David, if you REALLY want to try to make MAMP PRO work for you with Letsencrypt -- as opposed to switching to Abyss Web Server -- then, assuming that MAMP PRO is a purchased version that is properly installed in your "Applications" folder, and assuming that your document root is a subfolder in the "Sites" folder, then this is the Terminal command that should work:
sudo certbot certonly --webroot --webroot-path /Users/Your-Account-Name-Here/Sites/Your-Document-Root-Folder-Name-Here
Provide your Admin password when requested, followed by the domain name(s) you wish to register with Letsencrypt.
Let me know what happens after you try that.