AWS have published that their AMI with PHP 7.2 has sodium (cryptography).
Plainly (as at writing) it does not. There is no package available. There is no solution.
Have attempted installing etc, but it is a long way around and messy.
Instead, there is an excellent PHP 7.2 Polyfill, from the orignal authors - right here: https://github.com/paragonie/sodium_compat
Here's how to install it: