I apparently have V 17.10.60742 and the latest version is 18.0.0.6142 of 187MB. Whether I press update now or update later and schedule it, nothing happens.
I just get the reminder next time I start the app.
Since you have version 17 currently, what you are apparently being offered, version 18, is clearly a full version upgrade. So, is your Update Service still current? IOW, did you either buy your current version within the last 12 months or have opted, since the purchase, to renew and pay to maintain your Update Service?
If your Update Service is not still current I think that would explain why you see the message about an update but actually cannot install it.
OTOH, if your update Service is still current then I am really not sure why you are experiencing this problem!