No valid GUI code page for language & (system code page &)

What causes this issue?

The system was unable to set a valid GUI codepage for the logon
language.

System Response

Conversion of characters between the system character set and the
character set of the end user device will not function correctly.
Incorrect characters may be displayed. When characters are entered,
invalid data may be saved to the database.

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?

Contact your system administrator.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.