Hi,
Based on the above description, we're using SCOM 2019 (without any URs or hotfix) and the error message is as below:
The pool managing the instance is not available or the location monitoring object cannot be found
From the error message, our problem may relate to network settings, e.g., firewall port blocking or settings in ConfigService.config describled in below article:
http://jimmy-scom.blogspot.com/2017/12/the-pool-managing-instance-is-not.html
Note: this website is not from MS, just for your reference.
Hope the above information helps.
Alex Zhu
If the response is helpful, please click "Accept Answer" and upvote it.