Since you had this problem twice, let’s not disregard it. You say you read all the other related posts. But did you read this Guide?
https://forum.manager.io/t/installing-or-updating-desktop-edition-on-macos/7116
I think the most common problem for Mac users is trying to open the disk image that is downloaded directly. That won’t work. Open the disk image and drag the Manager icon to Applications. The Manager icon is not a single application; it is a bundle. Follow the procedures described in the Guide exactly and installation or update is foolproof.