Key/Index component & is a table

What causes this issue?

INCLUDE DDDOC_EXPLIZIT_KEYDEF_COND OBJECT DOKU ID TX
You defined a key component that is a table.

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?

Procedure for System Administrators

Remove this key component from the key definition and define a more
suitable key component in its place if this is necessary.

Error message extract from SAP system. Copyright SAP SE.