Planning plant is taken as the plant for the main work center

What causes this issue?

You have entered a <GL:main_work_center>main work center</> without
entering the associated plant.

System Response

The <GL:maintenance_planning_plant>planning plant</> that was entered
is taken as the plant for the main work center.

How to fix this error?

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

Error message extract from SAP system. Copyright SAP SE.