Source and target databases are required

What you might see

You may see: `source_and_target_database_required`, compare creation failure, schedule precheck failure, or snapshot queue failure.

Why it usually happens

- The mapping was saved before selecting source or target databases.
- A connection default database is blank.
- The mapping points to stale connection IDs.
- The database was renamed or removed.
- A schedule references an old mapping.

How to fix it

1. Edit the mapping.
2. Select a source database and target database.
3. Save the mapping.
4. Re-run schema discovery if the database list has changed.
5. Update any schedules that reference an older mapping if needed.

How to avoid it next time

Use explicit database selections in mappings and validate schedules after modifying connections.

SQL to check

```sql
SHOW DATABASES;
SELECT DATABASE();
```