Thanks to all that posted possible "resolutions" to this issue. I have a new Samsung Galaxy Book Pro with Windows 11 and have been experiencing the same issue with a blank activation screen on TT H&B 2021. Technical assistance was useless. Here is what I did: I opened TT and got the blank activation screen. I then kept opening it until I got the actual activation screen to pop up. Although the cursor does not show up in the activation key box, I entered it anyhow...WITH NO DASHES, just the 16 alpha/numeric code. I then clicked activate and when nothing happened, I waited a minute or so and then clicked activate again, and got the registration screen. Evidently, even though the cursor does not show up in the activation key box, it is taking the keys as entered, we just don't see them. I hope this helps....
