I can’t updated to iOS 13. I erased all data to factory reset but still won’t update

These are the iOS options based on iPhone Model --> Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

• iOS 13.7 —> 6S / 6S Plus, SE, 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max
• iOS 12.4.8 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, iPad Air, iPad Mini 2, iPad Mini 3 and iPod Touch (6th generation)

if you have a 6 plus, you cannot upgrade beyond 12.4.8. it is not compatible, there is nothing wrong.. there is no problem to fix.

I am seeing that every other iPhone has an update to IOS 14 but my phone iPhone 6+ is stuck at IOS 12. Even though I tried everything my phone still won’t update and its saying my phone is up to date which it’s not. I don’t know how else to fix this problem

Your iPhone 6 doesn't have the processing power to support iOS 13. If you somehow got iOS 13 loaded on your phone, you would brick it.