I’ve gotten that one on occasion when creating a new character from scratch. It helps when the “Character name” field matches something in the first line of the “Character description/personality/instruction/role”. For example. “Jane” in the character name field, and in the description I have:

Jane Anne Doe:

Fullname: Jane Anne Doe
Nicknames: Jane (preferred), Janet, Anne, Annie (she hates this one)

etc.

Usually though I only get that error on the very first run of a newly created character. Then it vanishes after first use.