If any Media becomes unavailable during Analysis it'll be canceled.
This should only happen on rare cases where files are moved or external media has been disconnected.
You should restart analysis and make sure media is connected and available.
REAPER's default behavior is to make media offline when it's not in the foreground.
If you're using REAPER and would like analysis to continue while it's in the background you should change the default behavior as follows:
- From REAPER's preferences, go to root Media tab on the left.
- Make sure 'Set media items offline when application is not active' is unchecked.