The features of a web browser, such as adblock, have no bearing on the use
of the Mac App Store to obtain the correct update or upgrade for your macOS.
Within the App Store you should be able to get the download update/upgrade.
Or if you have a working browser, there is a link to look into the App Store item
from Apple Downloads: Apple - Support - Downloads
For information -and- download button, see first blue link. For direct download
of .dmg, see second item of same name with 'download' link in blue beneath it.
Download
If you have Safari browser, you may be having other issues; so it may be well to
restart the computer to see if that helps clear up some of them. If you have gone
and installed third-party utilities or cleanmymac/mackeeper, etc, uninstall them.
Perhaps this issue with the Browser should be a priority. 😐