My daughter's teacher told her that the sentence "The data was undeniable." is not grammatically correct but has no idea what is wrong. Can anyone help?

I think it is incomplete.
The data is undeniably wrong (or correct) would probably be better. But I'm no expert.

The word data is the plural of datum. Even though many treat it as singular (which is acceptable these days), some still prefer to see it as a plural. Therefore, "The data are undeniable" might be what they're looking for.

There is more on data is/are here.