[ElronTheElder]

Yup, bought a new Nexus 6, it was delivered yesterday.
Fi app told me this version couldn't install on my phone.
Phone was running 5.0 LNX07M. Believe the app needs 5.1+ to run.
FDR with/without sim didn't force an OTA update...nothing did.
Looked high and low for an OTA so I wouldn't have to unlock my new phone.
Couldn't so I updated it by flashing it to Marshmallow.
It runs well.
After I flashed my phone found a listing of OTA downloads, of course.
A great resource to have.
Here they are:

[REF] Nexus 6 Stock OTA URLs | Nexus 6 | XDA Forums

[npaladin-2000]

Lemee guess, got it from BlinQ on eBay? Same thing happened to me. Couldn't flash an OTA, had to unlock the bootloader and flash it to Marshmallow. And oopsie, gee wiz, forgot to re-lock the bootloader, now absent-minded of me.

[MattKerwin]

Yeah...the Fi app won't install unless the phone is unlocked

[ElronTheElder]

Yeah...the Fi app won't install unless the phone is unlocked

?

Need 5.1 or higher then it will install if locked.

EDIT: XDA is on the lookout for OTA MRA58N update url. This is the security patch for Fi users. [If somebody captures it Post it on the forum.]

[mogelijk]

An even better source for factory images for all Nexus devices is the Google Developers site.

[ElronTheElder]

An even better source for factory images for all Nexus devices is the Google Developers site.

Of course.
However.:
OTA updates are easier for beginners to install.
Don't wipe data.
And don't need to have an unlocked bootloader.