Message number 999999 reached. Log is full

Application log: Display logs:
For technical reasons, only a limited number of messages can be part of
an application log.
An application log (an entry in the table BALHDR) can have a maximum of
999999 messages (up to 999999 entries in table BALDAT). Both these
tables are associated with each other using the field LOG_HANDLE.

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

Solution:
Reduce the number of messages in each application log or check whether
it is necessary to generate this many messages.

Error message extract from SAP system. Copyright SAP SE.