Sadly there is some problems on Apple's end, it looks like it's happening as a result of recent changes since the Big Sur update. We will continue to work on a long term solution, but in the meantime, first ensure that your software is completely up to date (Big Sur 11.2.1) - this has worked for some, but not all. It also didn't work for me, but give it a go first.
Following that, there is a workaround using a xattr. This is not the ideal solution, but the only working one right now. There are risks associated with using a xattr. Although this workaround will get you playing, educate yourself prior to using this command on potential dangers/issues to make an informed decision. Use this workaround at your own risk, PRO or PRO Staff are not liable for any complications caused as a result.
I have created a video guide on how to do this, here is a step by step:
Once the client is downloaded, expand the PROClient_mac.zip and move the PROClient.app into the Applications folder.
Press ⌘ and space, to bring up the Spotlight.
Search "Terminal" and press Enter to open.
Input the following command and press Enter to run.
sudo xattr -rd com.apple.quarantine /Applications/PROClient.app
You will be prompted to enter your User Password, enter this and Press Enter.
The change is now live. This will only apply to the specific app you have targeted with the xattr command. If you uninstall the client, you will need to repeat this process upon redownload, and for any additional clients. For any additional clients, you will need to amend the xattr command to include the name of the client - to do this amend the "PROClient.app" element of the command to the name of the application you wish to apply the command to. e.g. "sudo xattr -rd com.apple.quarantine /Applications/PROClient2.app"
Close the Terminal.
Visit the PROClient in your Applications, and Open/Double Click to run.
If you have any questions, please let me know and I will do my best to help you.
As mentioned in Private Messages also - the first Mysterious Ticket is rewarded at 0.5%, and the second is rewarded at 3% of contribution, as opposed to 1%.
Yellow's post in updated mac client not working was marked as the answer