"I don't know any culture that requires people to eat meat!"

You have not studied any indigenous cultures where hunting and meat are sometimes part of tradition and rituals.

Are they required?

I don't belong to the cultures so I won't say it's a hard yes, but when hunting is a major part of your culture and meat is important to your survival (thinking Arctic people) the message to go vegan is very Colonizer.

The other big problem with the whole "Give up meat to save the environment!" is that not all vegan-approved foods are grown and shipped in an ethical way.
You need to eat local first and see what you can get locally before switching over.

If your vegan diet is 90% shipped overseas... that's not eco-friendly.

It it's harvested by people who exploited and underpaid (and really so many veggies are) ... that's not helping.

If you want to save the planet through dietary changes support the local CSA, nearby by farms, local farmers markets, and raise as much of your own food as you can.
Start local and see what's available.

Lots of people try to go vegan/vegetarian for the environment and don't know enough about local growing seasons and eating seasonally to do that in a sustainable way.

Just eating seasonally and locally so you aren't paying to have food shipped from another continent is enough to help encourage sellers to shop locally.