During Instagram login using iOS (swift 3) app I am getting error which is below:
{"code": 403, "error_type": "OAuthForbiddenException", "error_message": "You are not a sandbox user of this client"}.
Please help me to understand and solve above issue.
You're application has not yet been reviewed by Instagram and approved. Therefore, you must develop in Sandbox Mode. It's a pain in the butt, let me tell you.
The gist of Sandbox mode is that you have to get a few of your friends on instagram to log into the Developer Site and accept your Sandbox Invites. Then, you'll be able to use your API client to view their profiles.
Instagram basically creates a little sandbox version of Instagram where only you and your friends (that have approved the Sandbox Invite) exist.
IMO, this is such an obstacle for developers. If you PM me, I'd be happy to accept a sandbox invite for your application because I know how annoying it can be to have to ask your friends to do this just so you can make your video and submit you application for approval (I am @mikeghen on Instagram).
I highly recommend taking the time to thoroughly read through these documents detailing Sandbox Mode and Permission Review.
Also, definitly verify that your application falls into one of the use cases for Instagram third-party applications. See "Valid Use Cases..." section on the Login Permissions documentation. If you don't fit into one (exclusively one from what I understand), then you have to go back to the drawing board. IMO, its best to fit your application into the use cases here because then people will actually use your application and Instagram will let you have access to the API.
Since locking down their API in June 2016, Instagram has made it much more difficult for dev by introducing the Sandbox. However, if you can get 5-10 friends of yours on Instagram to accept your Sandbox Invites, then it's really smooth sailing (until you try to get your application approved :P). Good luck! PM me for more.