What partner integration are you using that’s throwing this error? You’ll need to reach out to them since it’s there code that should be setting the grant_type when refreshing the token. 
I’m all set! Opencart!
Thank you.
Glad to hear it’s working as expected.
@shondat711 how did you fix the problem?
Hello Bryan, Im experiencing the same issue.
How you can solve it?
Sorry, Im new in Square, my account as well is new.
Are you experiencing this with one of our partner integrations?
I keep getting that same error.
How the heck do you fix it??
What’s your application ID?
I’m experiencing the same problem in OpenCart version 2.
After entering My application ID and Secret key on the extension setup page I click CONNECT.
I’m redirected to Sqaure login.
I login and then get redirected back to the extension setup page.
I don’t receive green connection notice and my ID and Key are not saved.
The log on the square site shows the following error:
“message”: “missing required parameter ‘grant_type’”,
“type”: “bad_request.missing_parameter”
Help!
If this is with OpenCart I’d recommend reaching out to them. The integration your using might be an older one prior to grant_type being added in the OAuth flow.