Rachl, I am happy to explain the solution that worked for me. I am not a tech expert but just followed the steps that others suggested in this thread. I do hope that you manage to solve this issue or find a work around. It is affecting productivity for many people. Unfortunately Microsoft Forums do not have a way to reply one on one. If I share any contact info here, it's for all the world to see.
I followed the solution marked "Answer" on this thread I deleted the "olk.exe" file and replaced it with another file with the same name that was just an empty text file. Others have used the new Outlook remover tool to disable Outlook.
Up until yesterday I was able to use the old Mail app without any problems. Then, when I launched Mail I got this message.

I clicked on Open outlook, the window disappeared and nothing happened. So I am still able to continue to use the old Mail app. In the top right corner there is a small rotating circle showing that Outlook can't launch. The new Outlook can't launch because I disabled it. It's trying to launch the empty file, "olk.exe".
I also noticed that the old Mail app is no longer available in the Windows store. However, the current and previous versions can still be downloaded here:
https://windows-mail-and-calendar.en.uptodown.com/windows/versions
I plan to continue to use the old Mail app as long as I can. However, I understand that one day I may not be able to.
In the meantime, on another thread I am following, some have switched to "Wino Mail" available on the Windows Store. Some say it is similar to the old Mail app. It's free for up to 3 mail accounts or the pro paid version without restrictions. There is also Thunderbird, Mail Bird and many others. If you search for "Alternatives to mail and Calendar" you will get lots of hits.
I hope this helps. If you still wish to disable the new Outlook and see if you can get the Old Mail app to work, there are many here who can help. But the Mail and Calendar app's days appear to be numbered. It may well be the time to move on to another app.
Kind regards
Chris