I googled Uffizi museum and found a link that worked. I’m all set!
http://www.uffizitickets.com/
Just FYI: the website you used is not the official ticketing site - it's a 3rd-party- and you're very likely paying a premium price. For instance, I clicked on a random January date for an advance, timed-entry adult ticket and it comes up with a price of 44.00 €. That same ticket booked through the official site is listed as 29€. Of course if you can't access that official site...
https://www.uffizi.it/en/tickets
Really, as official ticketing sites that are malfunctioning drive travelers to 3rd-party sites that charge more - sometimes a LOT more - please do report your issue to the contact I provided the link for.