Unable to upgrade my iPhone 6

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

You need to Identify your iPhone model - Apple Support

• iOS 14.4 —> 6S / 6S Plus, SE (1st Gen and 2nd Gen), 7 / 7 Plus, 8 / 8 Plus, X, XR, XS /XS Max, 11,11 Pro /11 Pro Max, iPhone 12, 12 Mini, 12 Pro, 12 Pro max
• iOS 12.5.1 —> Final release supported on iPhone 5S, iPhone 6/6 Plus, iPad Air, iPad Mini 2, iPad Mini 3 and iPod Touch (6th generation)
• iOS 10.3.3 —> Final release supported on iPhone 5C and iPad (4th generation)
• iOS 10.3.4 —> Final release supported on iPhone 5 and iPad (4th generation) Wi-Fi + Cellular
• iOS 9.3.6 —> Final release supported on iPhone 4S, iPad 2 Wi-Fi + Cellular, iPad (3rd generation) Wi-Fi + Cellular, and iPad Mini (1st generation) Wi-Fi + Cellular
• iOS 7.1.2  —> Final release supported on iPhone 4.
• iOS 6.1.6 —> iPhone 3GS and iPod Touch (4th generation) only
• iOS 4.2.1 —> Final release supported on iPhone 3G and iPod Touch (2nd generation)
• iOS 6.1.5 —> Final release supported on iPhone 3GS, and iPod Touch (4th generation)
• iOS 3.1.3 —> Final release supported on iPhone (1st generation) and iPod Touch (1st generation)

What version do you have.

The latest version compatible with your phone is 12.5.1