Non-executable element changes in a dynpro line

What causes this issue?

By overwriting existing elements and/or creating new elements on a line
of the layout editor, you created a situation where the system either

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

cannot create a unique assignment to existing elements on the line
or
cannot create the required space for a permanently visible possible
entries key

System Response

Not all the requested changes have been performed.

How to fix this error?

Before saving/generating, check that the assumptions made by the system
correspond to your wishes. To do this, look at the element list or
select the <LS>Attributes</> function for the critical fields. If you
do not agree, leave screen editing without saving or generating. Then,
restart the editing process and divide your complex changes on one line
into several smaller changes.

Error message extract from SAP system. Copyright SAP SE.