Icon & is not a valid pushbutton icon.

What causes this issue?

You want to convert a text field containing an icon to a pushbutton.
Not all icons can be use anywhere on the screen. The icon currently
assigned to the field text is not allowed as a pushbutton icon.

System Response

The system will not convert the field text to a pusbutton as long as
the icon is assigned to the field text.

How to fix this error?

If you want to convert the field text to a pushbutton, 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.