В воскресенье она так и не смогла доспать, потому что домофон зазвонил слишком рано.

For days of the week, Russian often uses в + accusative to mean on a certain day.

So:

• в воскресенье = on Sunday
• в понедельник = on Monday

Here воскресенье is a neuter noun, and its accusative form looks the same as the nominative, so the form does not change visibly.

In the past tense, Russian verbs agree with the subject in gender and number.

• он смог = he managed
• она смогла = she managed
• они смогли = they managed

Since the subject here is она, the verb must be смогла.

Так и не adds a special nuance: something was expected, intended, or hoped for, but in the end it still did not happen.

So:

• не смогла = she couldn’t
• так и не смогла = she never did manage to / she still couldn’t in the end

This is a very common Russian pattern:

• так и не пришёл = he never came after all
• так и не понял = he still didn’t understand

So here так и не смогла доспать suggests she wanted or expected to finish sleeping, but that never happened.

Доспать means to sleep the remaining amount, to finish sleeping, or to get the rest of one’s sleep.

It is different from some related verbs:

• спать = to sleep
• поспать = to sleep for a while
• выспаться = to get enough sleep
• доспать = to sleep the rest of the time you still wanted or needed

So не смогла доспать means she could not finish the sleep she still had left.

After смочь (to be able, to manage), Russian normally uses an infinitive:

• смогла доспать = managed to finish sleeping
• не смогла прийти = wasn’t able to come
• смогли сделать = managed to do it

So смогла is the finite verb, and доспать tells us what she managed or failed to do.

Домофон is the intercom or door-entry phone/buzzer used in apartment buildings.

In this sentence, it is the thing that rang too early and woke or disturbed her.

The prefix за- often marks the beginning of an action.

So:

• зазвонил = started ringing
• звонил = was ringing / rang / used to ring, depending on context

Here зазвонил fits well because the sentence describes a sudden event that interrupted her sleep: the intercom started ringing too early.

Потому что means because and introduces a subordinate clause. In Russian, a subordinate clause is normally separated by a comma.

So the sentence is divided into:

• main clause: В воскресенье она так и не смогла доспать
• subordinate clause: потому что домофон зазвонил слишком рано

That is why the comma is required.

Because рано is an adverb, not an adjective.

• рано = early
• слишком = too

Together, слишком рано means too early.

Adverbs do not change for gender, number, or case, so the form stays the same.

Yes. Russian word order is flexible, and changing it changes the emphasis more than the basic meaning.

This sentence begins with В воскресенье to set the time frame first: On Sunday...

You could also say something like:

• Она в воскресенье так и не смогла доспать...

That would still be grammatical, but the emphasis would be slightly different. The original word order sounds natural and puts the time expression up front.