Make sure that at least one processing node is reachable and that you have granted the current user sufficient permissions to view the processing node (by going to Administration – Processing Nodes – Select Node – Object Permissions – Add User/Group and check CAN VIEW PROCESSING NODE). If you are bringing a node back online, it will take about 30 seconds for WebODM to recognize it.
If you haven’t already fixed this, I ran into the same thing recently. User permissions was not the issue in my case, despite the hint. I stopped everything, ran “./webodm.sh update” and then started again with “./webodm.sh start”. It worked fine from there. (You may also need to delete the existing processing node from the list under “Processing Nodes”. Then restart. I recall doing that also, but I’m not sure it was part of the fix.)