Type change between text field and input/output field is not possible

What causes this issue?

One of the following has occurred:
You are editing a screen element with type input/output field
(template) and have entered a non-allowed character for the first
character of the template (not '_' or '?'.
You are editing a screen element with table text field (key word) and
have
entered a text containing only underscores '_'
entered '?' as the first element of the text.

System Response

If the text you entered begins with '_' and you are editing a text
field (key word), the system regards the leading underscore as
meaningless and deletes it.
The system then decides the type of the screen element from the first
remaining character:
'?' Obligatory input/output field
'_' Input/output field
Other Text field
The first character that you entered does not correspond to the current
type of the element.
Since implicit type switching is no longer supported from Release 3.0
onwards, the system does not accept your entry.

How to fix this error?

Check your entry. If you really want to change the element type, you
need to delete the element and recreate it.
For pushbuttons and boxes, you can also change the type by setting and
resetting the 'Output field' attribute. You do not then need to delete
and recreate the element.

Error message extract from SAP system. Copyright SAP SE.