If it’s still not working, there’s a few reasons why. You either set the date or time on the website wrong, which I know you said you didn’t, but the first time I made a Fireside I thought that too and it turns out I messed it up.
If that’s not it, you could be on the wrong account. You can only open gatherings with the account you set it up on.
The only other reason I can think of is that the new Fireside rules for this expansion have messed with the ability for non-established taverns to make Fireside Gatherings, but I don’t think that’s true.
If the date and time really is right for your Fireside, try making a new Gathering for the closest hour or half-hour from now, which could allow it to work.