I moved from windows to macbook recently. I have the latest microsoft 365. The import/export function was greyed out in my case too. I follow the community instruction and change the new outlook to the legacy outlook in order to import the mails from the old PC. After that, I successfully import the mails from the old PC. However, the mails are not integrated into my existing mail folders, and instead they are hidden in a backup folder under "on my computer". I understand there are a lot of compatibility problems between windows and mac, but I found that this is increasingly unacceptable since the same bug can be traced to a few years ago and yet microsoft cannot or would not fix it.
The old mails are now in a hidden box, a little better than in the trash box. However, I don't think this is the level of program service that microsoft should do or accept. I expect the transfer can be fully integrated into the existing mail folders with different email accounts.
Secondly, if the new outlook does not support the function, fix it or do not put it out as a product or feature. It is wasting the consumer time to look for the reason why the latest and updated version of the program is not working. If you want to test it, there is a type of programs call "the beta". Do not call it "new outlook", call it "beta-outlook" and so people can expect more bugs they may face.