If you encounter the above error when attempting to install LockDown Browser on a Mac, please review this article: https://support.respondus.com/hc/en-us/articles/4409607170459-What-are-the-computer-requirements-for-installations-of-Respondus-LockDown-Browser-
The "digital signature" error will occur when attempting to install LockDown Browser on a Mac that is running an operating system older than macOS 10.13. You'll need to upgrade to at least macOS 10.13 in order to use LockDown Browser on your Mac.
If you cannot upgrade to the latest version of macOS in the App Store, we recommend going to "How to get old versions of macOS" at https://support.apple.com/en-us/HT211683
However, if an upgrade is not possible, you'll need to use a different computer to take exams that require LockDown Browser.