I created a sandbox account and tried to get AUTHORIZE_CODE but could not.
Following is the link:
https://squareup.com/oauth2/authorize?client_id=sandbox-sq0idb-iL_RUhdQOtzprJyfdUHdXw
The document followed: https://developer.squareup.com/reference/square/oauth-api/authorize#query__property-client_id
Hi @dila, welcome to the forums!
It looks like you are using a production url with a sandbox application id, which will not work. You should use squareupsandbox.com... instead of squareup.com.... So I think this URL should work:
https://squareupsandbox.com/oauth2/authorize?client_id=sandbox-sq0idb-iL_RUhdQOtzprJyfdUHdXw
Let me know if that helps or if you have additional questions.
its not working. You can click on the link that you are suggesting.
aright got it, the error is different now, suggesting to do other things , Thanks man
now it says 
To start the OAuth flow for a sandbox account, first launch the seller test account from the Developer Dashboard. (Few days ago i was able to launch the seller test account, but now i do not find to do the same)
You do need to launch the Sandbox Seller Dashboard first, which can be found on this page: https://developer.squareup.com/apps at the bottom under Sandbox Test Accounts, you need to click “Open” and leave it open; in another tab, try the OAuth link again.