What causes this issue?

There are only 50.000 free numbers of a total of 999.999 for Business
Navigator object identifiers left over for the currant prefix.

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?

Please run Report SBMCV000 with a Prefix of your choice. Be aware that
you chose a prefix in your name range or you will receive an error
message.
The prefix is valid for all process modelling activities in the current
client. It is necessary to allow transports from one system to another.
If different prefixes are used in source and target system, there are
no possible transport conflicts because of identically named objects.
We recommend not to use prefixes as 'ZSAP' or 'ZCUS' or 'Z001' ...
because accidential identities of prefixes in one company would be very
likely. Instead you should use more unusual prefixes similar to 'Z7TQ'
or 'ZO9X', if there are no safe general naming conventions in your
company.

Error message extract from SAP system. Copyright SAP SE.