Please enter material number or account assignment category

What causes this issue?

You have entered neither the
<DS:GLOS.3526C075AFAB52B9E10000009B38F974>material number</> nor the
<DS:GLOS.3528EA71E8A0556FE10000009B38F983> account assignment category
</> in the item. An item representing a material without a
<DS:GLOS.3526BF30AFAB52B9E10000009B38F974>master record</> (i.e. no
material number is specified) or a service must have an account
assignment category.

How to fix this error?

Enter a valid material number.
If you wish to procure a material without a master record or a service,
enter a suitable account assignment category for the item.

Error message extract from SAP system. Copyright SAP SE.