Delete the name & of the OK_CODE field first

What causes this issue?

You tried to convert the screen to an include screen (subscreen).
However, the OK_CODE element of the screen already has a name.
The OK_CODE field is not available in a subscreen. Instead, function
codes are passed back to the OK_CODE field of the main screen. If you
used the name of the OK_CODE field in the modules of the subscreen, it
would refer to an undefined value.

System Response

The system cannot convert the screen into a subscreen.

How to fix this error?

In the element list, delete the name of the OK_CODE field (by
overwriting it with spaces). Check as well whether you have used this
name in the screen modules to evaluate the OK_CODE, and change the
program logic accordingly. You can then return to the screen attributes
screen and repeat the operation.

Error message extract from SAP system. Copyright SAP SE.