It’s normal if occasionally you don’t get an erection the moment you want to. It happens to every guy once in a while, for whatever reason. It can for instance be caused by stress, too much alcohol or not wanting to make love (with that person). It can be embarrassing, but it’s normal.
There is a problem if your penis fails to get firm time after time. This may be due to psychological factors (feeling guilty, fear of intimacy, depression, anxiety etc.) or be physical in nature (hormonal problems, diabetes, drug use, neurological problems etc).
A rule of thumb is that it is psychic if you have morning erections and get erections masturbating, but don’t get a hard on when you make love.
If you regularly fail to get an erection at moments you might expect it you better go to your GP. You have to discover the cause in order to find a solution.

Myths promoting erection problems
Fear of failure
Treatment for psychic causes
Treatment for physical causes
Viagra

I met a girl at the disco. But when we wanted to make love I did not get a hard on. How come?