No allow button in security and privacy preferences
That's because it's not meeting the guidelines for the possible install sources.
Now, the remainder of this answer is going to assume you're a legitimate user of the box with Admin permissions (or access to an admin account on the box):
Easiest fix: when you launch the installer, hold down the option key. That should offer you a prompt to override the security restrictions.
More permanent, global fix: Launch your terminal, and enter the following:
sudo spctl --master-disable
This will provide you henceforth the ability to install untrusted app (read: not notarized by Apple) from any source.
Running
sudo spctl --master-enable

will reverse the process.