I have a plugin wich syncs Lightroom to WordPress. (WPLR Sync) I wanted to upload a folder over the Lightroom plugin, but I get the error "Can't update this collection. An internal error has occurred: wplr.lrpugin/WPLRSyncExportServiceProvider.lua:66 4: attempt to compare number with nil" https://snipboard.io/vJYez4.jpg
"Can't update this collection. An internal error has occurred: wplr.lrpugin/WPLRSyncExportServiceProvider.lua:66 4: attempt to compare number with nil"
Your screenshot shows the collection you're trying to publish is currently sorted by Custom Order. LR 10.0 introduced a bug with custom-order collections that was fixed in a later version. So double-check which version of LR you're running by doing the menu command Help > System Info. If you don't have 10.3, do Help > Updates.
If the problem still occurs with 10.3, select the collection in the Publish Services panel and do View > Sort > Capture Time. Then try republishing it.
While 10.2 fixed that bug, it's possible that bug hasn't completely fixed issues with publishing-service collections. (The programmer who implemented the change in LR 10.0 was clearly not as skilled as they should have been.)
A couple of next steps to narrow down the issue and hopefully work around it:
1. Tediously select each published collection, including "Photos", in the WP/LR Sync service and do View > Sort > Capture Time. Perhaps when you try to publish, the plugin is accessing these other collections.
2. Try creataing a new collection in the publishing service and publishing it.
3. Try creating a new instance of the publishing service with a collection containing a few photos.