Go is static type so it is very strict with data type itself. []int64 is a slice (keeping it simple: "imagine it as a basket full of arrays to form “dynamic array”) while [size]int64 is an array. Since they are 2 different data types, Go will throw an error.
To fix this, you can easily convert any existing array into slice using the slice bound [:].
a := [size]int64{ ... }
x := a[:] // x is now []int64 type