You probably haven't provided usage of the function. So compiler was confused:

let sumTwoSmallestNumbers = 
    Seq.sort 
    >> Seq.take 2 
    >> Seq.sum // The type 'System.IComparable' does not support the operator 'get_Zero' 

But if you add something like code below, then type will be inferred as int64.

sumTwoSmallestNumbers [|5L; 8L; 12L; 19L; 22L|] 
|> printfn "%A"

Error message gives you a hint though: "Consider adding further type constraints"