I used " I cannot understand Beth", which is the idiomatic English for "I am not understanding". Why isn't this accepted? Would, "I don't understand" be more acceptable?
I'd say "I don't understand" would be a closer match. But by phrasing it this way it is very much in the moment. Later you might understand Beth, and earlier you might have understood her, but at the moment you are not understanding Beth
"I can't understand Beth" wasn't accepted.