Issue Description
The operation could not be completed.(com.apple.softwareupdateservices.errors error 31) when updating iOS devices via Mass Action command.
Troubleshooting Steps
This is normally due to the devices not being on, connected to wifi, and plugged in or having 50% or more battery.
Note: iOS devices would have to be connected to wifi in order to update; allow more data on 5G must be selected to update iOS on 5G.
More Resources
5G - https://support.apple.com/en-us/HT211828
Updating iOS devices - https://support.apple.com/en-us/HT204204