No character set was maintained in TSYST for & system

What causes this issue?

A character set was not maintained in table TSYST for the active SAP
System &V1&.

System Response

The word processing system is not able to format texts as it does not
have the necessary information about the system character set. This is
found in TSYST.

How to fix this error?

Maintain the system character set for the active system $ in table
TSYST.

Error message extract from SAP system. Copyright SAP SE.