Only one supplier posting possible for net posting

What causes this issue?

You have chosen a document type that allows direct clearing of cash
discounts in stock or expense accounts. In this case, you are not
allowed to create more than one supplier line in a document.

How to fix this error?

Terminate the processing of "New item: supplier". Post the individual
supplier lines with the relevant items in separate documents. You can
create links between the individual documents via the reference and
header text in each document header.

Error message extract from SAP system. Copyright SAP SE.