I'm am 13 years old (teen) and I am unable to keep an erection for more than 1 Min. Its about 0.4 inch when its not erected what could be causing this?

Nope that is not causing it. Most likely the fact you are 13 is why.

Your thirteen years old. Your just beginning puberty. Considering your penis is only half and inch long when flaccid (non-erect) I would assume your penis size is roughly an inch to an inch and a half or so. That size isn't quite ready to hold an erection because the tissues haven't fully developed yet. Just give it a little more time and wait for it to grow and develop. You'll be able to enjoy practices soon enough.

Have patience!

~~Apple Pro~~