Она не только не вставила диаграмму на слайд, но и забыла выделить заголовок.

It is the Russian equivalent of not only ..., but also ....

So:

• не только не вставила диаграмму на слайд
• но и забыла выделить заголовок

means that two things are being combined:

1. she did not insert the diagram onto the slide
2. she also forgot to highlight the title

This structure is very common in Russian and works much like English:

• Он не только опоздал, но и забыл документы.
  = He not only arrived late, but also forgot the documents.

In your sentence, the first part is negative, which is why it looks a little more complicated.

Because they do two different jobs.

• The first не is part of the fixed pattern не только ..., но и ... = not only ..., but also ...
• The second не negates the verb вставила = did not insert

So:

• не только = not only
• не вставила = did not insert

Together:

• не только не вставила = not only did not insert

This is normal Russian. English also does something similar:

• She not only didn’t insert the diagram, but also forgot to highlight the title.

Because it agrees with она.

In the past tense, Russian verbs show gender in the singular:

• он вставил = he inserted
• она вставила = she inserted
• оно вставило = it inserted
• они вставили = they inserted

So вставила tells you the subject is feminine singular.

The same thing happens with забыла:

• она забыла = she forgot

Вставила is perfective because it refers to a single complete action: inserting the diagram onto the slide.

Even though the action did not happen, Russian still often uses the perfective here when talking about a specific expected result:

• Она не вставила диаграмму на слайд.
  = She didn’t insert the diagram onto the slide.

This sounds like a concrete, one-time failure to complete the task.

If you used не вставляла, that would sound more like:

• she was not inserting it
• she never inserted it
• she didn’t engage in that process

So the perfective is more natural when talking about a specific task in a presentation or document.

Because it is the direct object of вставить.

The verb вставить takes a thing that gets inserted. That thing goes into the accusative:

• вставить диаграмму
• вставить картинку
• вставить таблицу

For feminine nouns ending in -а, the accusative singular usually changes to -у:

• диаграмма → диаграмму

So:

• диаграмма = diagram/chart
• вставила диаграмму = inserted the diagram

Because на слайд expresses direction or destination: onto the slide.

Russian often distinguishes:

• на + accusative = motion onto / to
• на + prepositional = location on

Compare:

• вставить диаграмму на слайд = to insert the diagram onto the slide
• диаграмма на слайде = the diagram is on the slide

So in your sentence:

• на слайд answers where to?
• на слайде would answer where?

Normally, no.

In presentation/computer language, Russian idiomatically says:

• вставить на слайд
• добавить на слайд
• поместить на слайд

That is because the slide is viewed as a surface or page-like space, so на is the natural preposition.

You might say:

• в презентацию = into the presentation
• на слайд = onto the slide

So:

• вставить диаграмму в презентацию = insert the diagram into the presentation
• вставить диаграмму на слайд = insert the diagram onto the slide

It is the common pattern:

• забыть + infinitive = to forget to do something

So:

• забыла выделить = forgot to highlight

Here:

• забыла is the past tense of забыть
• выделить is an infinitive

This structure is extremely common:

• Я забыл закрыть дверь. = I forgot to close the door.
• Она забыла отправить письмо. = She forgot to send the letter.

So the sentence is not saying she forgot how to highlight the title. It means she failed to remember to do it.

Here выделить is perfective, and that fits the idea of a single intended action with a result: highlighting the title.

So:

• забыла выделить заголовок = forgot to highlight the title

That sounds like one concrete task she was supposed to complete.

After забыть, both aspects are possible, but the meaning changes a bit:

• забыла выделить = forgot to highlight it
• забыла выделять can sound like forgot that she was supposed to be highlighting things regularly, or forgot the general practice/habit

In this sentence, the perfective is the natural choice because it refers to one specific action in one specific slide/task.

Because it is the direct object of выделить.

The verb выделить takes an object:

• выделить заголовок
• выделить текст
• выделить слово

For masculine inanimate nouns like заголовок, the accusative singular is the same as the nominative:

• nominative: заголовок
• accusative: заголовок

So the form does not change, but the case is still accusative because of its role in the sentence.

Russian often omits subject pronouns when the verb already makes the subject clear.

So you could say:

• Не только не вставила диаграмму на слайд, но и забыла выделить заголовок.

and it would still be understandable in the right context.

However, она may be included:

• for clarity
• for contrast
• because the speaker wants to explicitly mention she

Since вставила and забыла are feminine singular, the verb forms already point to she, so the pronoun is not grammatically required.

It is part of the fixed correlative structure не только ..., но и ...

You should think of но и here as one unit meaning but also.

So:

• не только X, но и Y
  = not only X, but also Y

The и is not just a separate and added randomly. It belongs to the pattern.

Examples:

• Он не только читал, но и писал.
  = He not only read, but also wrote.
• Она не только пришла, но и помогла.
  = She not only came, but also helped.

Yes, Russian word order is flexible, but the original version is natural and neutral.

Original:

• Она не только не вставила диаграмму на слайд, но и забыла выделить заголовок.

This clearly groups the ideas as:

• not only did she fail to insert the diagram
• she also forgot to highlight the title

Russian can move elements around for emphasis, but with this structure, learners should keep the standard pattern:

• не только ... , но и ...

That is the safest and most idiomatic way to say it.