Why do I get 'object in use' error?
There are numerous reasons, most leading from the fact that some operations require exclusive database access (like adding a foreign key in earlier versions of Firebird). Things to try:
1. commit all your transactions and try
...if it does not work...
2. disconnect all other users and try. If you run Super Server you can check whether there are other connected users with your admin. tool.
...if it still does not work...
3. disconnect yourself, connect back and try
...if it still does not work...
4. turn off the computer and go to sleep. Fresh mind finds solutions faster.
This error can also show up while restoring the database. The best practice is to restore to a different yet unused filename, and rename the file once restore is complete.