You need to write it like this.
#import "@preview/letter-pro:2.1.0": letter-simple
#show: letter-simple.with(
sender: (name: "Charles", address: "123 Barker Street"),
font: "Charter"
)
letter-simple
is a function with argument font
to which you can pass the fontnames as a string
or an array
of strings.
Don’t hesitate to take a look at the Typst Tutorial on making a template.