"Mój tata jest wujek" is not a correct sentence. If you want to use the Nominative form "wujek", then it has to be "Mój tata to wujek". Which is a bit like "My dad = an uncle". Not the best option, although it's correct.
I wonder, perhaps it showed you "Mój tata jest wujem"? "wujem" is Instrumental of "wuj", which is an a bit old-fashioned word for an uncle. Also it sounds very similarly to a common swear word, so it's not the best choice.