Hello and welcome to the forum.
This is a well known bug in Qubes Updater GUI. It was reported here two weeks ago. It happens only if there is no known available update. This means your dom0 update should had been completed successfully and you have no pending update for any template.
A fix was submitted and merged around the same time. It is currently in current-testing repository as reported by Qubes project leader. You should receive it in few days once it is moved to current stream and system does the automatic background refreshing of update-availability checks.
p.s.: It is still possible to use qubes-vm-update and qubes-dom0-update command line utilities.