Field &-& required as a key field

What causes this issue?

You tried to delete a mandatory field.
Certain views and matchcode objects must contain at least all the
primary key fields of the included tables which are not simultaneously
foreign key fields.

System Response

You cannot delete the field.

How to fix this error?

Leave the mandatory fields in the view or matchcode object.

Error message extract from SAP system. Copyright SAP SE.