Uninstalling SQL Server Cumulative Update Issue
Why are you uninstalling a Cumulative Update?
If you no longer have the CU, download it again, then (you'll have to stop the current setup process first) run:
&gt; C:\whatever\cu-filename.exe /x c:\temp\cu-files\

Then start the uninstall over again, and when you get that dialog, hit Browse... and point it at c:\temp\cu-files.
It may find what it needs in the extracted CU files, but you may have to wait to perform this change until you have access to the 2017 image. (Any edition will do, I expect, so you could download the evaluation ISO for example.)
But let me ask again: Why are you uninstalling a Cumulative Update?