Using Classic Outlook on Windows for personal email, calendar, and contact management
Just as FYI ... I was having the license issues with my Basic EOL email address. I finally got everything to work by going into settings and making my free outlook (MSA) account (the same account I use to login in here, xbox, etc, etc - all the MS services - but more importantly - it is also my family 0365 account) as the default ("Set as Primary Account").
After doing that - I was able to add all my email accounts - Basic Exchange Online accounts, and other Work (E5 Licensed) accounts.
Source : How licensing works for work and school accounts in the new Outlook for Windows (microsoft.com)
If and when, the New Outlook will allow my basic Exchange account to be the default for email and events - things will be perfect; right now, I have to change it from my MSA to my E1 account.
Edit - I went to a different computer - and added my E1 account and had no issues; then went back to my primary computer - and removed my MSA account .. and everything is working (for now).