My girlfriend told me she's never been able to cum even when masturbating. What should we do?

The other day, before I ate my girlfriend out, I asked her if there was anything specific that she liked or wanted me to do. She told me she had no clue because she's never been able to cum even by herself. On top of that, it was my first time giving head and her first time receiving it, so we're both kinda lost at sea. It went on for like 30 minutes. There were times when she was feeling pleasure but it was sorta off and on and in the end she didn't cum.

Is it just a trial-and-error/experience thing? I really wanna be able to make her finish but we don't know what we're doing. Also we're long distance so we won't get to practice often. Any advice?