From your description, do you mean when trying to create a public live event, the option of "Public" is grayed out?
If this describes your situation, please go to Teams Admin Center > Meetings > Live events policies, choose the policy that is currently applied to the users, change the setting of "Who can join scheduled live events" to "Everyone" instead:
The change may need some time to take effect, so you can check the result maybe the next day. Per my test, it can work properly:
More information about setting up for live events can be found at Set up for live events in Microsoft Teams.
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