Welcome to Android Central! The listing on Metro by T-Mobile specifies up to 256 GB, so it should work. https://www.metrobyt-mobile.com/sho...ile-REVVL-4-Plus-64GB-Steel-Gray/610214664877.
Where did you get this card from? Does it work in other devices? There's always the possibility that the card is dead or counterfeit.
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U1/U3 is the speed the card can run at. Can run at - if the phone asks it to. It can also run in a slower environment.
The phone handles SDXC - that should work up to 2 TiB. (They can't implement a "64GB SDXC - it either supports to 2 TiB, or it's not SDXC.) I'd contact SanDisk - their online support is pretty good. A SanDisk Extreme card carries a lifetime warranty (that's a minimum of 30 years - Germany limits "lifetime".) Try SanDisk microSD Tech Support - upper right corner, Chat Live. At worst they'll replace the card.