TBC - Bug on "Send to Addon" button

I saw posts about this from 2017 so it might be related. Anyway…

I choose “SEND TO ADDON” on the website and get the following error:

I tried doing what it said but it wasn’t clear. What I did instead, however, was select a slot and clicked “Blank This Slot”.

After that I did RE-OPTIMIZE and then “SEND TO ADDON” worked.