If I go to turn it on, it turns back off either instantly or within a few seconds.
Any suggestions on what I might need to look into?
I've had this problem since I got McAfee along with my laptop, late last year.
Sometimes, I think Real Time Scanning is still functioning as a warning page will pop up if I browse onto certain websites, but I can't be sure.
That last warning is caused by McAfee WebAdvisor which is a browser add-on designed to make surfing safer.
Regarding the real-time scanning perhaps if you ran the Virtual Technician it might find and fix whatever is wrong: https://mvt.mcafee.com/
If it can't fix something then I suggest going to Control Panel > Programs and Features and uninstalling all McAfee products.
Reboot if asked.
Run the McAfee Consumer Products Removal tool (MCPR), reboot.
Reinstall from your online account.
If troubles persists then contact Technical Support, it's free of charge by phone or online chat and linked on the lower part of the Virtual Technician page
(see "Contact Support").