Only account owners or administrators in Zendesk or UserVoice are able to connect those helpdesk solutions to Sprout Social at this time; light agents or regular users cannot connect these accounts to Sprout.
Whoever connects the account to Sprout will be the person whose name appears as the ticket creator/requester in Zendesk/UserVoice.
If you are an administrator or owner in Zendesk or UserVoice and still having connection issues, make sure you're using the correct link to sign in. Some users have reported an SSO redirect/looping with login, so please check to see that your URL is correct.
Note: You must be the Account Owner of your Sprout account to connect Zendesk or UserVoice.