After I installed the latest version, QuickTime is not working and it is asking me for the Pro version. What can I do?
Goto apple's website and download the free Quicktime Player from there and re-install the application.
Yeah, I also think you should reinstall your QuickTime Player which might be a old version.
Have you considered to change another 3rd player, like VLC, or free Macgo Mac Media Player. If your installation is not working, this is a choice.
In order to fix the issue, remove your version of QuickTime using Add or Remove Programs feature (Control Panel > Uninstall a program), run CCleaner or AusLogics Registry Cleaner to make sure that there are no leftover registry keys in your system and reboot your computer. After that, navigate to Apple Downloads page, click the blue Download Now button, save the file to your computer and install it following the on-screen instructions.