The first table may not be altered.

What causes this issue?

You tried to move or delete the first table in a join.

System Response

This operation is not allowed.
You cannot change the root table of a join in a functional area of the
SAP Query afterwards.

How to fix this error?

Define a new functional area with another root table.

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.