It is compatible with android devices and is advertised as such "Samsung's T7 Touch is compatible with PC, Mac, Android devices, gaming consoles, and more.", as are all HDDs and SSDs.
If you're having an issue with file transfers then the issue lies elsewhere.
Does it show up as a storage option or usb device when you plug it directly into your s21?
I can see that it has a fingerprint lock, if its anything like Windows Bitlocker encryption, it requires the device its plugged into to support the software to decrypt the drive. It could be that the software for the fingerprint scanner just isn't compatible with mobile devices. This is common for drives that require the device to do the deception/encryption rather than built in hardware encryption within the drive itself.