Item category &3 not allowed with document type &1

What causes this issue?

The combination of <DS:GLOS.3526BFD0AFAB52B9E10000009B38F974>document
type</> <ZH>&V1&</> and <DS:GLOS.3526C02AAFAB52B9E10000009B38F974>item
category</> <ZH>&V3&</> entered cannot be used because it has not been
defined in Customizing for Purchasing.
<ZH>Example</>
You have created a purchase order of the document type "standard PO"
with reference to a contract. The contract item you are referencing has
the item category <ZH>M</> (material unknown) or <ZH>W</> (material
group). This item category was adopted in the PO.
If the combination of document type "standard PO" and item category
<ZH>M</> or <ZH>W</> has not been defined in Customizing for <ZK>
Purchasing</>, you must use a valid combination of item category and
document type.

How to fix this error?

Check your input and change the item category if necessary.
<ZH>Note</>
If you wish to create a stock transport order (with item category <ZH>U
</>), you must invoke the function <LS>Purchase order -> Create ->
Stock transfer</> or use the order type <LS>stock transport order</>.

Error message extract from SAP system. Copyright SAP SE.