Authentication oauth2.0

[It It]

I sent a get request tohttps://api.assembla.com/authorization?client_id=_client_id&response_type=code
 and I get the response {"error": "invalid_request :: \"redirect_uri\" missing"}.
Next I added redirect_uri and got the code.
Then I sent a request to https://_client_id:_client...@api.assembla.com/token?grant_type=authorization_code&code=_authorization_code substituting my Application identifier and Application secret instead of _client_id and _client_secret, and instead of _authorization_code I got the code from the previous request. I get the following response
{
    "error": "invalid_client",
    "error_description": "The client identifier provided is invalid, the client failed to authenticate, the client did not include its credentials, provided multiple client credentials, or used unsupported credentials type."
}
What am I doing wrong?