Had this same issue, and none of the fixes worked.
I was able to resolve it by manually editing the /root/mailinabox/management/ssl_certificates.py file and adding the following two lines on line 334:
"--agree-tos",
"--email", "you@email.com", # use your email here
So it ends up looking like so:
Save the file and re-run the script using ./ssl_certificates.py
Once the SSL certificate is successfully provisioned, you can remove the two extra lines you added.