I am running version 2021.005.20155 for Windows. Adobe Acrobat DC Pro can use the Rich Media tools to create a PDF with video but it fails to play that video. The same file Pro created can be opened and viewed in the current Adobe Acrobat Reader with no problems. It can also be created and viewed on Adobe Acrobat Pro on the Mac. The version of the Mac did not match the Windows version. The Mac version is 2021.005.20048. I assume this is a bug. We tried resetting the Preferences by hand as well as using Repair Installation under the Help menu on the Windows system. All ten computers on the Windows system had the same exact issue.
Hope you are doing well and sorry for the trouble. As described you are unable to play the video in PDF using Adobe Acrobat Pro 21.05.20048 installed on Windows OS.
What happens when you try to play the video, do you get any error message? If yes, please share the screenshot of the same for a better understanding.
Please try to reboot the computer once and check. If it doesn't work, please try to turn off the protected mode for testing (Win Only) Go to Edit > Preferences > Enhanced Security > Uncheck 'Enable protected mode at startup' Turn off the protected mode and Uncheck enhanced security > Click OK and reboot the computer. Note: Please turn on the security after testing to avoid and security risks.
As you have already tried resetting the Acrobat preferences and repairing the installation with no success. Please try to create a new test user profile with full admin rights in Windows and try using the application there and check.