I use Strategy Tester to test all the alerts coded for my clients since many years without any issue ...
Oh I see now. Just opened the MTF indicator you added the alert in. This kind of MTF coding (not your alert!) is the only one that doesn't work in tester. I changed the way I code MTF few years ago so they can run smoothly in tester ...
MT4/MT5 EAs/Indicators/Alerts coder