You cannot bring up the Pivot Editor if the report (grid) you are looking at has more than 2000 columns.
You can probably fix your report by going to "Reports > Edit Report", and uncheck the box "Pivot Replicate Name" at the bottom of the Customize Report dialog.
When you uncheck that box, the report will contain all the same data as before but it will have fewer columns and more rows.
You might run into other problems when you try to use the Pivot Editor later on. If you send me your Skyline document and report definition I might be able to walk you through whatever you are trying to do.
One way to send the report definition would be to include it in the Skyline document. To do that, you would go to:
Settings > Document Settings > Reports
and check the checkbox next to the report that is having trouble with the pivot editor.
After that, you can use the menu item:
File > Share
to create a .zip file containing your Skyline document and supporting files including extracted chromatograms.
If that .zip file is less than 50MB you can attach it to this support request.
Otherwise, you can upload it here: