I was trying to train my character and the trainer told me I don't have enough stamina. Stamina isn't a characteristic. Why can I train my character?

Maybe the character didn't have enough training points?

I have 2 training points so I don't think that is the problem. I also have enough money.

I had no problem training other characters. Just the Avatar. I still don't understand the reference to "Stamina".

I think sometimes you need at least 3 training points (depends on the trainer).
The trainer may have said stamina but he possibly just means you need more experience.

This.
Have a look here for detailed info on U7 trainers (with stat raises and costs).
Ultima dialogue (and manuals), even regarding trainers, is always "in character", so some information in game may sound vague.

If a trainer trains you for 3 attribute points (2 str and 1 dex for example) he won't train you unless you have 3 training points. 1 for each point of attribute increase

Thanks to everyone for your responses.