So I only failed the naming of the isotope right

Yes, that's correct. You correctly identified the atomic number, mass number, and the element.

For the isotope question, you correctly calculated the new mass number as 37. Your only error was in the way you wrote the isotope's notation and name.

Instead of $^{17}\text{Cl}^{20}$, it should be:

• Chlorine-37 (name)
• $\boxed{^{37}_{17}\text{Cl}}$ (notation)