is the package still listed under Packages → Management or nor?
When restarting the instance, are there any error messages in the server.log, indicating why WmJDBCAdapter can not load correctly?
My only concern here is that the title is about 9.12 but the screenshot you are sharing is for version 10.15 IS (and 10.3 adapter). You are not trying to install 9.12 adapter to 10.15 IS by any chance, are you? JDBC adapter version 10.3 is the latest version for IS version 10.15 as far as I know. So version 10.3 adapter should be the correct version to install 10.15 IS.