i get erections but i cannot cum !

Hello,
Welcome to the ehealthforum and I am really glad to help you out.

If you are having regular erections and had ejaculation during sex, then relax as there does not appear to be anything wrong with you. The white dry substance is drying of smegma released which occurs normally in males.

What you are facing is sex related anxiety. This usually happens in initial stages and the problem will disappear over a period of time.

It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.