Perhaps its because your primary account is licenced as business premium which is the highest possible subscription? I only have a home account for my primary Microsoft account and I do not intend to increase the subscription just to enable my second email address. There is only me, myself and I using it at all times so that would be a total waste of money! I pay for my second email address through GoDaddy which corresponds with the .au domain I purchased through them, hence it becomes classed as a 'work' email which is not permitted without an upgrade in licencing.
All in all, it's all about more money, more money! I've been using Microsoft office for years, there was no such thing as subscriptions in those days, these were introduced by big tech to FORCE people to pay out on a regular basis for the use of their software. When everything was on disc, you had a CHOICE to upgrade and IF you wanted to, you bought the new version outright.
The only option i can think of trying to incorporate my second email address onto Outlook desktop is to transfer it away from GoDaddy to another server so it's removed from the 365 licence. Perhaps that may then work, I really don't know. Alternatively i just move to another emailing platform. If i do that i would drop my Microsoft Office subscription altogether, there are plenty of decent free alternatives for Word and spreadsheets. For now I'm using the previous outlook version on which my second email still functions. It's familiar to me so I guess i will just stay with that as long as I'm able to.