That message is usually triggered if more than one update process are trying to run at the same time.
Processes which lock the resources include things like manual updates, background update checks, package managers etc.
A possible clash is an attempt to run an update from the Welcome Screen just after boot.
Unless set to 'never', background checks may be running at any time but commonly run just after boot.
Depending on the speed of your connection and the size of the updates, the resource may be locked for significant time.
This condition will self clear once the background check completes so sometimes all that is needed is to wait.
Before running a manual update, any package managers need to be closed to avoid a clash.
If the problem does not self-resolve it is possible that something (like an interrupted update) may have broken the mechanism and more investigation will be required to clear the lock.
