Question

Why is mi abuelo es argentina incorrect

Answer 1

Mi abuelo es argentina is incorrect because abuelo means grandfather, who is obviously a male, and the adjective argentina is used to denote feminine gender because it ends with -a. Instead, if you want to correct this sentence, you should write the adjective in masculine form (ending with -o).
So, it should go like this:
Mi abuelo es argentino.