Error: The requested action requires that the target document is the frontmost document.

I had this error occur and solved it with these steps.
1. A smart object was being edited in Illustrator that wasn't saved. I saved the smart object and closed it in Illustrator.

2. Save the Photoshop file, re-open it, and try the export again.

Hi @paul.os 

Thanks for reaching out.

Since when are you noticing this? If you try a different export workflow, does this same thing happen?

Try updating Photoshop to the latest version; check here: https://helpx.adobe.com/photoshop/kb/uptodate.html#:~:text=How%20do%20I%20check%20for,See%20Update%20Creative%20Cloud%20apps.

Let us know if that helps. 

Ranjisha