When you are trying to add a new org to Flosum, occasionally an error msg will appear that reads: "Org already exists with organization ID."
This may indicate that the org previously existed in Flosum and was deleted, but that some settings were left behind after the deletion.
In order to fix this error, go to "Custom Settings" and open "Authorize Details."
If the org ID that you are trying to add exists there, delete it.
Once you have completed those steps, try to add the org again. The error message should no longer appear, and you should be able to add the org successfully.