Any software installation or update process (like a manual update, a background update or running a package manager (like Synaptic) requires exclusive access to update resources and will lock them while running to prevent another update process starting. If one update process has locked the resources and a second one tries to start, the second will not be able to get the resource and will generate a message like this.
Usually all that is needed is to wait until the first process completes (or close Synaptic) to release the resource.
Depending on the speed of the connection and the size of the updates, the resource may be locked for a significant time. This is especially the case after a fresh installation as many updates may be being processed.
If the problem does not self-resolve when the first process is finished, it is possible that something (like an interrupted update) may have broken the mechanism and more investigation will be required to clear the lock.
