Assigned number & is the last available number

What causes this issue?

In using the last assigned number, you have reached the end of the
number range.

How to fix this error?

Contact your system administrator so that the number range can be
extended. Until this is done, no further material master records can be
created.

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

Error message extract from SAP system. Copyright SAP SE.