iPhone data transfer issue

The issue here is that your iPhone 6s is updated to a higher iOS version than your iPhone 6. You cannot transfer data from a higher iOS version to a device that has the lower iOS version. The other issue is that your iPhone 6 cannot be updated to a an iOS version greater than iOS 12.5.5.

Give the conditions listed above, you will not be able to accomplish what you have set out to do in the manner that you have intended.

What you can do is backup your iPhone 6s to a computer as shown here ➡️ Backup your iPhone on a computer and then use your iPhone 6 until you rectify the issue(s) on your iPhone 6s. Once the iPhone 6s has been fixed, then you can ➡️ Restore your iPhone from the computer backup.

Axel F.

You cannot transfer data from an iPhone with higher iOS to another with lower iOS using any of the three processes listed below

iPhone to iPhone, there are three ways,

1. Quick Transfer --> Use QuickStart to transfer data to a new iPhone, iPad or iPod ...
2. Restore from iCloud Backup --> Restore or set up your device from an iCloud backup - Apple ...
3. Restore from iTunes backup --> Restore your iPhone, iPad, or iPod touch in iTunes on PC ...

However, try transferring data manually. One by One which is possible

1. Messages, Media, Voice Memo, Keychain --> Turn on Messages in iCloud & iCloud Photos, Voice Memo, Keychain, and your content is automatically stored in iCloud and is available on all devices that use the same Apple ID
2. Contacts --> Contacts are not stored on an iPhone nor on SIM, they are synced to  Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other. Use the email account that is in your old iPhone.
3. Notes, Reminders, Calendars --> You may sync all these from any of the email accounts that you have on your old iPhone. they can be synced using  Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other.
4. Any Specific Files --> Upload to Cloud Drives (iCloud Drive, Google Drive, OneDrive, DropBox, Flickr, box, AmazonDrive, Adobe Creative Cloud and Use Files app on your new iPhone too.
5. Apps & Settings --> In any way they have to be redownloaded using the App Store. Even you had used any of the 3 processes mentioned earlier. Apart from the Apps following will also not get transferred.
   1. Content from the iTunes and App Stores or PDFs downloaded directly to Apple Books
   2. Content synced from iTunes, like imported MP3s or CDs, videos, books, and photos
   3. Data already stored in iCloud, like iCloud Photos, iMessages, and text (SMS) and multimedia (MMS) messages
   4. Face ID or Touch ID settings
   5. Apple Pay information and settings
   6. Apple Mail data
   7. Apple Music
6. WhatsApp --> Take a backup of chats and media from within the WhatsApp App. Use the same Phone number as that of your earlier iPhone and activate the account in the new iPhone too. That's it.
7. Shazam Account --> Access your Shazams on all your devices or online - Apple ...

first backup on your computer and after restore your other phone

You are welcome.

Axel F.