Have had this on and off since way back in the early days of 5.x of Simple Press. Using the File Uploader Plugin, can not get attachments to give me the option to click to upload file.
Its definitely a javascript issue, here is the error, but I have been down this rabbit hole before and testing with turning off every plugin, changed theme to WP standard, cleared caches, etc. Was basically ignoring this in the 5.x Series and figured it would resolve itself when upgrading to 6.x last year.
Suggestions on more places to look.
plupload.full.min.js?ver=5.1.1:29 Uncaught TypeError: Cannot read property ‘can’ of null
at j.u.onready (plupload.full.min.js?ver=5.1.1:29)
at j.<anonymous> (moxie.min.js?ver=1.3.5:1)
at moxie.min.js?ver=1.3.5:1