Please check the following list of reasons this can happen.
Is the domain correctly whitelisted on the Trackbox?
If the domain is correctly whitelisted, but the page is using no subdomain (i.e funnel.com instead of www.funnel.com), try accessing it with www - this can solve some whitelisting issues.
Are the ai and gi parameters in the page URL relevant? Are you entering from an IP of an allowed country?
Was the affiliate assigned to the relevant campaign? Are they both active?
Make sure you have tested with no cookies and a fresh IP. Testing without cookies is most easily done by closing all current incognito pages in your browser and opening a new incognito window. Refresh your IP via the VPN.
Another reason for this response is that the lead arrived in Trackbox correctly, but could not enter any brand - you can check in LEADS to see if you see your lead under 'failure' (all relevant brands did not accept the lead) or 'unassigned' (some misconfiguration, such as cap limitations or restrictions, is preventing this lead from entering). Either way, if you can see the lead in your CRM as a failure, you can click the lead and check the 'Brand Response' tab to see what the error is.