An 'existing' address in TB is one that has been established with a mail provider, but not set up in TB. TB doesn't offer the creation of new accounts - it only provides access to ones that already exist with the provider, new or old.

Who is the provider for your account?

TB = Thunderbird. user: wolfepac2/Falisa, are you saying its a bug ? and/or, Are you saying TB should open-up a new browser-tab inside the TB itself, & then visit the online mail service provider's website, and allow the TB-user to create a new email account in the GMail/Outlook-Live/Yahoo, etc website ? if thats what you want, please look into this, but if instruction there is too complicated for you, then, use a Firefox or other web-browser software to go into a MSP (mail service provider) website, & create new email-account there, also create an App-Password for TB , aka App-Key , aka Secure-Mail-Key,etc ( if your MSP provides such option ). and after those, you can use that email-address, app-password/key, etc inside TB , to create a mail-account in TB , to access emails for that email-address.

if you consider this a bug, or, lack of feature, etc, then, do you want to submit this bug/problem/feat-request into mozilla/thunderbird bug-report website ? (you may start from here: https://bugzilla.mozilla.org/enter_bug.cgi#h=dupes%7CThunderbird )