Installation halted at 'Allow'

Hello Folks.
Could somebody please point me in the right direction for how I actually get past the 'Allow' phase in System Preferences on my Mac?
This morning, after quite some time of not using this program for various reasons, I attempted to install version 12 of P. Desktop, which, on failing with the exact problem described above, politely informed me that there were issues and I'd be much better off upgrading to version 13. Following the obviously good advice, I duly upgraded.
However, the exact same problem has appeared and I cannot get past the 'Allow' phase, as the button has no effect whatsoever.
I've added a screenshot of where I can actually get to in the process.
I'd be most grateful if somebody could point me in the right direction. If there is no easy way, does somebody have a detailed description of how to wipe the Mac of all traces of 12 and 13 so I can start from a clean slate?
Any help and advice is greatly appreciated.

Many thanks in advance and kind regards.

Stan

 

I'm sorry, you have pressed the Allow button in macOS preferences and nothing happens?

 

Hello Alex. That's correct. I get so far, and then no further. Do you have a solution to this problem? Many thanks. Stan

 

Stan, that is very strange because we do not control it. It is managed by Apple and i have no idea why this happens. Give me more time. I will try to check it in our lab. I'm sorry, but did you restart macOS after that and try to reinstall Parallels Desktop. What macOS shows in the Security Preferences after reboot? Does it have Allow button or not?