Could I have my money refunded for the months that I have been paying for the iCloud+ for my iPhone 6 Plus that the app can not be used on?
OR

1.
Check iPhone type & iOS version -> Go to: Settings -> General -> About.
If it is an iPhone 6 Plus -> iOS 12.5.5 is the last security updates -> released 23 september 2021.
2.
Here's an iPhone 6S (iOS 15.6), can make good use of iCloud, so don't understand why you can't use iCloud+ with your iPhone 6S Plus.
See this Apple article: System requirements for iCloud
System requirements for iCloud – Apple Support (UK)
3.
If it is an iPhone 6S Plus, your iPhone is eligible for an update, to iOS 15.6, you can check by reading this Apple article: iPhone models compatible with iOS 15.6 -> iPhone models compatible with iOS 15.5 – Apple Support (UK)
Then go to: Settings -> General -> Software Update
See the following Apple articles:
Update your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch - Apple Support (IN)
If your iPhone, iPad or iPod touch won’t update
If your iPhone, iPad, or iPod touch won't update - Apple Support
You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC
Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)
If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder
Update your iPhone, iPad, or iPod touch with the Finder - Apple Support
You need about 7 GB device storage and good Service & Wi-Fi.
If they are On, turn Off Hotspot & VPN.
Could I have my money refunded for the months that I have been paying for the iCloud+ for my iPhone 6 Plus that the app can not be used on?
1.
No, is not possible.
Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.
If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.
You pay monthly for iCloud storage.
The subscription runs as long as you don't downgrade.
You cannot deactivate/pause the subscription for 1 month, but you can downgrade to free (5 GB).
The iCloud + subscription will be terminated depending on the start date.
If you start on August 1 and are going to cancel now, the subscription will stop on November 1, a monthly term is used.
2.
You can downgrade your iCloud subscription, read carefully what happens if you downgrade iCloud, see this Apple article: Downgrade or cancel your iCloud+ plan
Downgrade or cancel your iCloud+ plan - Apple Support (PH)
3.
Get more help:
Call Apple in your region see: Contact Apple for support and service
Contact Apple for support and service - Apple Support
or -> Contact Apple Support -> Get Support (apple.com)