As Skydiver 119 has stated.
The 7-year old internal hardware in the iPod Touch 5th gen is too old and too underpowered to handle any new iOS versions.
If Apple allowed this, the 5th gen iPod Touch would run so poorly that you’d be signing up here whining, ranting and complainng and looking for anyway to make the 5th gen iPod Touch fast, again with no way to revert/downgrade to the previous, better performing iOS version it used to run.
If you need a better iPod Touch, the 6th gen iPod Touch is still available from Apple to purchase that CAN run the latest iOS versions.
https://www.apple.com/ipod-touch/
Time to upgrade from a 7-year old iDevice.
Also, if you can still find them, the iPhone SE, 32 GBs storage models are still selling at a really low price on some online retail sites for well under $200 and in some cases, around $150.
Those are much cheaper than a new iPod Touch and has all the functionality of the iPod Touch, plus all of the iPhone features.
The iPhone SE can just be used over WiFi, if you want. No cellular service plan is needed if you do not want it.
You have the option of getting a cellular plan, though.
Plus, the iPhone SE works with Apple Watch and watchOS, giving you even more options/features that aren’t available on an iPid Touch.
Good Luck to You!