New Item Request BPM does not allow adding or modifying Serial

Summary:

The business requires a Serial approval flow for the New Item Request.

The problem is that the PBM does not allow you to modify it to Serial or add a new one, as is the case in other flows (Contract).

The details are in the attachment.

Content:

In the Contract PBM it can be modified to convert it to Single, Parallel, Serial

But in the BPM of New Item Request it does not allow me to modify.

The business requires an approval flow in Serial, but it is not available in the standard flow and does not allow modification.