Impotence problems for 21 y/o, what to do?

Get a workup: At your age, if you're not getting super-hard with your favorite pictures / ideas / videos / partner(s), you need a proper medical workup, including checks for hemochromatosis and prolactinoma. You're also fairly new to romance, and learning how your body responds in different situations -- if there's no organic basis, this will solve itself as you gain skill ; experience with the right person.

Lots of reasons: There are many reasons. Diabetes or other endocrine problems, nerve problems or damage, medications, drugs, alcohol to name a few. There is also the possibility of psychological issues. A visit to a doctor could help sort it out.