What does it mean double free detected in tcache 2 while using mpz?

Your code has undefined behaviour because you are trying to assign l from an uninitialised array str.

I’m guessing you got your functions confused and meant to write the opposite

mpz_get_str(str, 10, l.get_mpz_t());

That code assigns l to str.

Use the following code to work out how big str needs to be

size_t size = mpz_sizeinbase(l.get_mpz_t(), 10) + 2;