Double check what version of php-mysql is installed like this:
apt policy php-mysql
It should be the same as the version of PHP you are trying to update to. Alternatively you could try explictly installing the version you need, like this:
apt install phpX.Y-mysql
Where X.Y is the version of PHP you are trying to install. E.g. 7.3.
Sounds like php-mysql isn't installed?!
Double check what version of php-mysql is installed like this:
It should be the same as the version of PHP you are trying to update to. Alternatively you could try explictly installing the version you need, like this:
Where X.Y is the version of PHP you are trying to install. E.g. 7.3.