Hello,
this is a known problem, you could have a look at the following links, there are different solutions shown.
In the topic Krita 4.4.3 start up error - damaged take a look at the eighth posting. In the topic Damaged When Downloaded, your problem is also covered, and in the two YouTube videos linked below, their creators also promise to offer the solution, I hope for your sake that they are right 
How to fix app “is damaged and can’t be opened…” OS X Sierra
as well as
HOW TO FIX APP IS DAMAGED AND CANT BE OPENED, MOVE TO TRASH? MAC! PROBLEM SOLVED FAST AND EASY 2018!
Michelist
Sorry, I’m lazy today, it’s translated with help of DeepL.