Icon & is allowed only as a pushbutton icon.

What causes this issue?

You are editing a pushbutton that contains an icon and want to
reconvert the pushbutton to a text field.
Not all icons can be used everywhere on the screen. The icon currently
assigned to the pushbutton is not allowed as pushbutton of a field
text.

System Response

Conversion rejected.

How to fix this error?

If you want to reconvert the pushbutton to a field text, you must first
delete the assignment of the icon. You can do this on the "Attributes'
screen, or in the field list by overwriting the icon name with blanks
or cancelling it with '!'.

Error message extract from SAP system. Copyright SAP SE.