This UAC prompt only happens when not-in-admin mode tries to launch a program as yes-run-in-admin mode. You can test this by using this command to launch notepad in run-as-admin mode
/run -a notepad.exe
If mirc is not in run-as-admin mode, this generates the UAC prompt. However if mirc itself is running in run-as-admin mode, there will not be a prompt, and notepad would be running in run-as-admin mode without the UAC prompt, regardless whether using the -a switch or not. I don't know of a way to launch a program not-in-admin mode from mirc running in admin mode, so you might not wish to click 'yes' on that prompt, assuming it lets you launch mirc without being in admin mode if you click 'no'.
since it apparently wants to run in admin mode without having that box checked in the 'compatibility' tab of properties, try googling for:
unable to start program without run as administrator windows 10
there's several different links with different possible causes for your problem