Unable to increase length

What causes this issue?

You attempted to lengthen an element in a fixed position, but there is
not enough space for this to the right of the field. This is due to one
of the following reasons:

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

another element or subscreen is taking up this space
the right screen edge or box edge has been reached
the element has the "permanent possible entries key" attribute and the
space required for the "possible entries" icon is insufficient.

System Response

The field is lengthened only as far as possible.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.