I see two problems
Sonarqube 6.7.3 > Sonarqube 9.9 LTA is a BIG gap
The upgrade path is always your version > next LTA(S) > next LTA(S) … > LATEST (optional)
In your case 6.7.3 > 6.9 (IIRC) > 7.9.6 > 8.9.10 > 9.9.7
Did you use the correct update path !?
The historical downloads page has all versions but no 6.9.x
I once used the wayback machine for a similar case, see
As Sonarqube doesn’t support MySQL anymore, how did you convert from MySQL to PostgreSQL ?
Maybe the conversion (export from MySQL and import into PostgreSQL) is the cause for the database problems.
I’m not sure if you can use the Sonarqube db copy tool or if it only supports the copy between databases Sonarqube runs on.