Thanks soooo much agriesser! 
the utf8mb4_0900_ai_ci problem is finally fixed!
but sadly, I’m stuck at new error.
(maybe this will be a last tribulation)
I got this error.
cannot activate plugin: error initializing the DB: driver: mysql, message: failed to fetch applied migrations, command: select_applied_migrations, originalError: Error 1054: Unknown column 'name' in 'field list', query: SELECT version, name FROM focalboard_schema_migrations
So I remember this thread↓
The problem this person is having is similar to my phenomenon, but
What do you think? 
and, Thanks again for your kindness and massive help 