Recursion for phase &

What causes this issue?

You cannot assign the phase to another phase as its predecessor, as you
required. This is because this assignment could lead to the situation
whereby the phase is (directly or indirectly) its own predecessor.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.