Your external database must be a newly-created (or empty) database.（I can not connect mysql）

[Helen He]

I just want to try to link the database to a database that has data in it is not OK or mysql needs to be upgraded to 8.0(Or jira only write a new database and then write data to my database) 

[Mohamed Benziane]

Hi,

Welcome to the community

Try to link jira to an empty database, then update this new database with the old data.

[Nic Brough -Adaptavist-]

Or connect Jira to an empty database, so that it goes through the basic setup, then stop it, repoint it to the old database, restart it, and run a full re-index.