How to recover Mac Startup Security Utility?
The reason you can't access the Startup Security Utility is because it doesn't exist for your model of MacBook Pro.

More specifically, Startup Security Utility is only available on Mac computers that have the Apple T2 Security Chip. So far the only Macs that have the T2 Security Chip are:


iMac Pro
Mac mini models introduced in 2018
MacBook Air models introduced in 2018
MacBook Pro models introduced in 2018


For more information I refer you to the following Apple pages:


About Startup Security Utility
About the Apple T2 Security Chip