Only checkboxes, radio buttons and pushbuttons allowed in loop lines

What causes this issue?

A line of the loop block contains an element that is not allowed in
loops. Valid objects are templates, field texts, radio buttons,
checkboxes and pushbuttons.

System Response

Unable to generate loop block.

How to fix this error?

Leave loop block processing with <LS>Back</> and remove the element
from the loop block.

Error message extract from SAP system. Copyright SAP SE.