What causes this issue?

Either an expression or a message is assigned to each variable in the
task text. Variables are replaced when the task is created. You cannot
add an expression or a message to the task text if the related variable
is missing.

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?

Delete superfluous expressions or messages. Alternatively you can
enhance the missing variables (such as &1) in the task text.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.