As the error message indicates, there might be other catalyst projects which is under migration . Only one project can be migrated at a time under a Catalyst org which means there must be other projects under your Org which are under migration.

To resolve this error, you would need to contact the account Super Admin to find which project is under migration and either complete the migration or abort the migration completely. You can refer to this official documentation on how to find the Super Admin for your project.

You can ask them to follow the instructions mentioned in here for complete or abort the ongoing migration which should hopefully resolve this issue.