I believe what you are trying will not work. the database schema has been changed many times from 10.3 to 12.2. so importing the database as a whole won't work.
you could update your 10.3 system to the latest version that is supported on both platforms, install that version on your new server and then update from there to 12.2
if this is not an option there is a API Script for copying over certain parts of the database. however there are some things that need to be moved manually.
I have done this in the past so if you need consultancy let me know.