As you’re using a SmartApp instead of an OAuth integration project, you need to make the authorization accordingly, if you’re not using the SmartApp SDK, here are comments from other developers that have managed to authorize their SmartApps:
As you’re using a SmartApp instead of an OAuth integration project, you need to make the authorization accordingly, if you’re not using the SmartApp SDK, here are comments from other developers that have managed to authorize their SmartApps: