Problem simplified by removing fixed requirements

What causes this issue?

The optimizer was again unable to find a solution even though all
discrete constraints had already been removed.

System Response

Due to the profile settings for unsuccessful solution determination, all
hard constraints caused by fixed orders are now removed and the
optimizer makes another attempt to calculate a solution.

How to fix this error?

If you would like the optimizer to respond differently in this case, you
should change the settings in the optimizer profile.

Error message extract from SAP system. Copyright SAP SE.