Execution of 2 assortment declarations at same time is not allowed

What causes this issue?

You try to execute two assortment declarations at the same time.
The following document types are regarded as assortment declarations:
S = assortment declaration
T = key date declaration

System Response

The assortment declaration cannot be executed.

How to fix this error?

Change the consecutive documents of the main document type of the
recycling partner, since at least one consecutive document is of
type S or T.

Error message extract from SAP system. Copyright SAP SE.