ORA-01503 CREATE CONTROLFILE FAILED... while trying to create a database
First of all, you could check the newly created init/spfile (under; linux: $OB/dbs - or - win: $OB\database) if everything is set correctly (SID and Service Name, Datafile location...)

I also would advise you to check all your env. vars, the error says “Database already mounted” - it seems it’s trying to create a .ctl for your already-running database.

You can try this to start fresh;

Linux


copy your actual .profile in .profile_new
modify .profile_new as per your need ($OH, $ORACLE_SID,...)
./profile new
check you’re all set: env
start dbca again


Windows


open cmd as Admin
set ORACLE_SID=[your new SID]
set ORACLE_HOME=[your new $OH]
run dbca


Last thing, i would take a look at the oraInventory/oraInst.lok file, to check if there’s the old $OH set - if yes, move the file away (just to backup it) and restart the dbca.

Hope one of this solutions helps.