Golang，要附加的第一个参数必须是slice； 有结构

brand new to go here and trying to set up a simple rest server to get the basics down of go. I have my routes configured and am now trying to set up some basic POST/GET calls with just storing some items in memory. I am trying to set up a struct and just push a payload into memory and fetch for example. I am not sure how to fix this error, but here is what I have so far.

The activity setup (activity.go):

package main

type Activity struct {
    activityCgid      string     `json:"activityCgid"`
    title             string     `json:"title"`
    description       string     `json:"description"`
    availableDate     int        `json:"availableDate"`
    dueDate           int        `json:"dueDate"`
    navigationTypes   []string   `json:"navigationTypes"`
    containerCgi      string     `json:"containerCgi"`
    position          int        `json:"position"`
}

type Activities []Activity

And then in my repo.go, I just have this :

func RepoCreateActivity(t Activity) Activity {
    activities = append(activities, t)
    return t
}

and above it, I have :

 var activities Activity

To which go is pointing to and saying first argument to append must be slice; have struct (and the struct following). Would appreciate any advice as I have just begun learning go. Thank you for reading!

 var activities Activity

that should be

 var activities Activities