This article describes how to resolve a 403 Forbidden Error in Respondus LockDown Browser?

When attempting a quiz with the LockDown Browser, you may get an error blocking you from accessing the quiz if you are using a VPN. You will need to disable your VPN during your test attempt to be able to use the Respondus LockDown Browser in this case. Respondus does not have a workaround to use both a VPN and their browser.

Find their help article here:

Please be sure to use a secure network to take your quiz.