Not pair with iPhone issue

Was the watch paired before with the same phone? If so remove it from the phone's paired devices first and try again through the app

I had pair with this phone. Yesterday it pair work fine. Moreover, everything worked without problems for 3 years.

Also I removed all links with watch: in App, and Bluetooth link

Try one more thing. Turn wifi off in the watch, restart it and see if it pairs via Bluetooth again. Mine has fried its wifi circuit after an update and if it is turned on it causes all sorts of problems -  Bluetooth as well. 

By the way check what is the wifi version in the about screen

Unfortunately, it didn't help. The watch has no problems with wifi, and bluetooth. I recorded a video to show what my problem looks like.

I also note: once again I reset the settings to the factory for watch and phone

My video: Garmin Connect App: Pairing Failed Message When Adding a Device Fenix 6

Can you try to pair the watch to another phone? Just to see which device has the issue

Edit: Just watched your videos. Try one more thing. During the pairing process instead of connecting to the device suggested manually select your device from the list of all available devices

I also thought about it, but also without success. I tried to connect the watch to my wife's iphone, she has a more fresh phone firmware and a fresh version of the garmin connect app

Also tried to manually select the watch model from the list of devices - without success

Also tried to change the Internet connection on the phone: another Wi-Fi or even through the mobile cellular - it did not help

Very strange error, never had such problems before

I'm tried connect another old my watch: Garmin vivoactive 3. Its successfully connected to the phone