I understood some basic Excel skills, but I did not know any advanced functions Excel had.
I wanted to know how to use Excel.
Therefore, I bought a book to look at what Excel could do.
Is the correct?
I bought a book on Excel to see what advanced Excel skills I could learn.
As a stand alone comment, that sentence would sound better in the present perfect tense to me as well.
But this was an exercise to fit various facts into one sentence, and given that all the facts were in the simple past, there's no reason to make the resulting sentence anything but simple past.
I've seen at least one other learner say that.
They know that they can't use present perfect when there IS a specific time reference, but somehow think they MUST use it when there is NO time reference.
That latter part, of course, is not true.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
The first post sets the context with three sentences in the past- I wouldn't use the present perfect there because of the context supplied by these. It's not a sentence in isolation for me.