The item cannot be added to this block

What causes this issue?

No more sub-items can be added to this main item.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

In Customizing for sales document types, you can define different
increments for the numbering of main and sub-items. Check these
settings.

Error message extract from SAP system. Copyright SAP SE.