Tengo una duda importante sobre el informe.

Because Spanish expresses this idea with tener (to have), not to be.

So:

• Tengo una duda = I have a doubt / I have a question
• not I am a doubt or I am doubtful in this sentence

This is very common in Spanish. Compare:

• Tengo hambre = I’m hungry
• Tengo sueño = I’m sleepy
• Tengo una duda = I have a question / doubt

Even when English uses to be, Spanish often uses tener.

Duda literally means doubt, but in everyday Spanish it often means:

• a question
• an uncertainty
• a concern
• something you are not sure about

So Tengo una duda importante sobre el informe can mean something like:

• I have an important question about the report
• I have an important concern about the report
• There’s something important I’m unsure about in the report

In real conversation, tengo una duda is extremely common when someone wants clarification.

In Spanish, adjectives usually come after the noun.

So:

• una duda importante = an important doubt/question
• el informe final = the final report
• una idea buena is possible, though una buena idea is more natural

With importante, the normal position here is after the noun: duda importante.

Putting the adjective before the noun is sometimes possible in Spanish, but it can sound more literary, emphatic, or stylistically marked. For a normal sentence, una duda importante is the natural choice.

Because importante is an adjective with the same form for both masculine and feminine nouns.

Examples:

• un informe importante
• una duda importante
• unos informes importantes
• unas dudas importantes

What changes is mainly the number:

• singular: importante
• plural: importantes

So the feminine noun duda does not force a change from importante to a different ending.

Sobre here means about / regarding.

So:

• sobre el informe = about the report

You could also hear:

• acerca del informe = regarding the report
• con respecto al informe = with regard to the report

But del informe would not mean the same thing here. Del is de + el, and it often means of the report or from the report, not about the report.

So:

• una duda sobre el informe = a question about the report
• una parte del informe = a part of the report

El informe means the report, so it refers to a specific report that the speaker and listener already know about.

• el informe = the report
• un informe = a report

If you say:

• Tengo una duda importante sobre el informe
  you mean a particular report

If you say:

• Tengo una duda importante sobre un informe
  it means about a report, perhaps one not yet identified

So the definite article tells us the report is specific.

Yes. That changes the meaning slightly.

• Tengo una duda importante = I have one important question/concern
• Tengo dudas importantes = I have important doubts/questions/concerns or several important concerns

The singular often points to one main issue.
The plural suggests more than one issue, or a broader sense of uncertainty.

Both are correct; it depends on what you want to express.

Yes, it is very natural in Spain.

A speaker in Spain might also say:

• Tengo una duda sobre el informe.
• Tengo una pregunta sobre el informe.
• Hay una cosa del informe que no entiendo.
• Tengo una duda importante con el informe.
  This can also be heard, though sobre el informe is very clear and standard.

A small nuance:

• pregunta focuses more on a direct question
• duda focuses more on uncertainty or something not understood

In many situations, though, they overlap.

Yes, although the original order is the most neutral and standard:

• Tengo una duda importante sobre el informe.

You could also say:

• Sobre el informe, tengo una duda importante.

This puts extra focus on sobre el informe. It sounds a bit more marked, but it is still correct.

Spanish word order is more flexible than English, but the basic order here is the most natural for everyday use.

A simple pronunciation guide:

• Tengo → TEN-go
• una → OO-na
• duda → DOO-tha in most of Spain, or DOO-da in areas with seseo
• importante → im-por-TAN-te
• sobre → SO-bre
• el → el
• informe → een-FOR-me

Stress falls on:

• TENgo
• DUda
• imporTANte
• SObre
• inFORme

In standard Spain Spanish, z and soft c are pronounced like the th in think, but this sentence does not contain those letters. The word duda normally has a regular d sound, though between vowels in fast speech it can sound softer.

Not necessarily. They are similar, but not identical.

• pregunta = a question
• duda = a doubt, uncertainty, or point that is unclear

Use pregunta when you clearly want to ask something.
Use duda when you want to express that something is not clear to you.

Compare:

• Tengo una pregunta importante sobre el informe.
  = I have an important question about the report.
• Tengo una duda importante sobre el informe.
  = I have an important doubt/concern/question about the report.

In practice, tengo una duda is extremely common when asking for clarification, especially in educational or workplace contexts.