Could not create pool & in ABAP Dictionary

What causes this issue?

The physical matchcode IDs (update types A, S, and P) for a matchcode
object are stored in a pool. This pool is automatically stored in the
ABAP Dictionary when the matchcode object is generated. The name of the
pool is formed from the prefix M_ and the name of the matchcode object.
In this case, the creation of the pool was not successful - for example,
because another pool already exists with the name chosen as standard.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Try to get further information about the error using the activation
logs.

Error message extract from SAP system. Copyright SAP SE.