Deleted marker &1 is used in equipment &2

What causes this issue?

You have deleted marker &V1&, which is used in equipment &V2&.

System Response

How to fix this error?

If you save the changes, the marker will be deleted. If you exit the
transaction without saving your changes, the marker will still be
available.
If you want to know in which technical objects the markers are used, the
following reports are available:
Change Functional Location (transaction IL05)
Change Equipment (transaction IE05)
On the selection screen of these reports, you can enter the linear
reference pattern as a selection criterion.

Procedure for System Administrators

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.