No periods for the single sentences. In addition, UX writing style would write these as:
Site address copied (I'd avoid 'URL')
Password copied
Username copied

For the 2 sentence toast, the punctuation is correct, but I would rewrite the text to be more user-friendly. "Can't sign in. Password not copied." or "Can't verify your identity. Password not copied." I'm not sure what "Authentication failed" means here. Isn't the user already signed in if they are able to access the password manager?

Thanks a lot for the guidance!

The single-sentence toasts are now clarified, I will implement the changes from #1.

For the two-sentence toast, I think "Can't verify your identity. Password not copied." is more appropriate than "Can't sign in. ..." The reauthentication is to verify that the user trying to view passwords is able to unlock the device, it does not serve the purpose of signing in to any account. I will therefore change the code to use "Can't verify your identity. Password not copied."

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/574f0bf33a61976d995c14e979d295f3bea8bb39

commit 574f0bf33a61976d995c14e979d295f3bea8bb39
Author: Vaclav Brozek <vabr@chromium.org>
Date: Wed Jul 19 10:00:48 2017

Adjust strings in passwords settings on iOS

This CL adjusts strings used in the passwords detail view, based on the
following guidance from UX:
  *  https://crbug.com/740379#c9  Shortening "Delete Saved Password" to just "Delete"
  *  https://crbug.com/740384#c11  Identifying the federation URL with "Signed In With"
  *  https://crbug.com/742290#c1  Improving the copy toasts: URL -> address,
    simpler phrasing and no full-stops for one-sentence messages

Bug:  740379 , 740384 , 742290 
Change-Id: I1eec2a44fd5d2acd59166988431832e88f8737e6
Reviewed-on: https://chromium-review.googlesource.com/577528
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487810}
[modify] https://crrev.com/574f0bf33a61976d995c14e979d295f3bea8bb39/ios/chrome/app/strings/ios_strings.grd

Site address copied 
Password copied
Username copied
Verified on iOS 10.3.3, 11.0 on iPhone 6 +, iPhone 7+, iPad Pro 12'5 on build 62.0.3202.29 Beta.