Multiple allocations are not allowed in the std. seq. or parallel seq.

What causes this issue?

You have carried out one of the following actions:
You checked consistency of component allocation in the routing. The BOM
item is allocated to more than one operation in the standard sequence or
parallel sequence.
You attempted to allocate a BOM item already allocated to an operation
in the standard sequence or parallel sequence.
Neither action is valid because a BOM item can be allocated to only one
operation in a routing. However, allocation of a BOM item to an
operation in an alternative sequence is the only exception.

How to fix this error?

First remove incorrect allocation(s) and then make the appropriate
allocation.

Error message extract from SAP system. Copyright SAP SE.