Sorry about that. Errors are uncommon as long as you're going through the normal upload process through GoToTraining Activities.
Did this just start happening? Has it worked previously for you?
This happens to me every time as well. I see the doc in the list, I select it and share it but then receive the same feedback message as shown. Is this because it was created by my G-Suite account?
The Google doc use in breakouts is spotty at best, especially with students spread across the planet with iffy internet connectivity. What I found works is:
1) Have the students use a back channel - skype works - for voice communications as chrome drops voice connectivity pretty frequently
2) If a student's connection drops they are thrown out of the group.
3) Once the break out is launched, if the student's connection drops they do not automatically go back into the group to which they were assigned. The instructor also can NOT drag and drop the students between groups after the breakout is launched. Instruct the students to keep refreshing their google chrome browser until they randomly wind up back in the right group.