The output is saying exactly what is happening and what you need to know!
As you are trying to use a %s verb which is meant to strings the output says that:
!s(int=0)
which means:
The value is not a string, but an integer.
Thus, if you want to know what to use instead take a look at the fmt package page https://golang.org/pkg/fmt/ at the "integers" table:
%b base 2
%c the character represented by the corresponding Unicode code point
%d base 10
%o base 8
%q a single-quoted character literal safely escaped with Go syntax.
%x base 16, with lower-case letters for a-f
%X base 16, with upper-case letters for A-F
%U Unicode format: U+1234; same as "U+%04X"
So you can use any of this verbs to have the output correctly represented.
Or as previous answers says, you can also use the %v verb which means:
"the value in its default format".