I just succeeded to download it on my computer, with the same version.
Did you check your internet connection ? If you don't have internet, or if TouchGFXDesigner cannot access to the server to download the TouchGFX Board setups, a red warning triangle should appear on the top left corner of the home screen.
Then close TouchGFXDesigner, go to "C:\Users\USERNAME\AppData\Roaming", and delete the TouchGFX-4.21.1 folder. After that open TouchGFXDesigner 4.21.1 again, wait 1-2 minutes and try again with H7B3-DK.