First, try restarting the computer. Next, turn off any virus or filter software, while you download.

If you still get an error, send an email to that effect to customer supoort. The time stamp on your email should prove that you tried to purchase during the sale, but could not.

… I have Print music from 2011 and want to upgrade tonight at the sale price …

… Mac OS High Sierra …

I suppose that you are trying to upgrade to (the “full“) Finale v25, right?

PrintMusic is not supported in High Sierra.

Your post isn’t making sense but let me try. PrintMusic 2011 will give you that error if you try to open it in High Sierra as will Finale 2011 (been there, done that).

You do not need a prior version installed to upgrade as long as your MakeMusic Account shows the prior purchase and registration of the license. The code should work and let you purchase, download and install — the license server will complete the upgrade when you register 25 using your new serial number (same serial# if you owned Finale in the past). This number should show up in your User Account and may be emailed to you.

If you are using the Chrome browser and having troubles with the upgrade process, switch to Safari or Firefox. 

Does that address your issue? If the problems (or misunderstanding of the process) have caused you to miss the deadline, contact Sales through the “Submit a request” link as has already been suggested.