i bought a universal iphone11 in the US but it gives me invalid SIM error in Australia. Why?

They lied. All iPhones sold by BestBuy are sold under the Flex policy, meaning they will lock to the first US carrier SIM used. They will not work outside the US.

The only source for unlocked iPhones in the US is an Apple store. The phone may work in Australia, but it will not be covered by the standard Apple warranty. Based on hundreds of similar posts here, BestBuy has a long history of being somewhat “less than honest” when selling iPhones.

Did you buy it in BestBuy?

An unlocked iPhone from the Apple Store will work everywhere.

The 8 is different from the 11 and has different LTE bands, meaning that one phone could work as a world phone while the other might not work worldwide. But still - an unlocked 11 should work as a world phone.

But it could also be very well be SIM-Locked to the US, meaning that only US SIM Cards will work, but i do not know. Best Buys Politics and how they operate enough so this is not confirmed. But that is possible, since you didn't bought from the Apple Store.

Also your iPhone's warranty is limited to the US, but that doesn't affect the functionality as a world phone. I always recommend to buy the phone in your country / locally to make sure you can use it and to avoid problems like it.