I recently came across an issue with Facebook Login after updating to iOS 13 and Xcode11. Login worked fine, all doc configuration is correct but two of my apps have this error now.
-canOpenURL: failed for URL: "fbauth2:/" - error: "This app is not allowed to query for scheme fbauth2
There is a similar question but it relates to iOS 10, and the issue clearly states that the info.plist configuration was incorrect, which is not the case here.
Upgrading to the current FBSDK should resolve the problem. iOS 13 requires the implementation of ASWebAuthenticationPresentationContextProviding, which was not available until FBSDKCoreKit 5.4.
FBSDKCoreKit/FBSDKCoreKit/Internal/BridgeAPI/FBSDKBridgeAPI.m was modified to support iOS 13's requirement to implement ASWebAuthenticationPresentationContextProviding in FBSDKCoreKit 5.4. (The current version is 5.8.) Versions prior will no longer work.
The needed change is posted here: https://github.com/facebook/facebook-ios-sdk/commit/6b061099f339ef0e8bde2e2c2163ef5a3c1b8340#diff-72593591275d63edfb1bfad837e4c32f
I was unable to upgrade due to a dependency with AWS, but I was able to patch the 4.42 version of the file based on the differences in the diff above.
The error -canOpenURL: failed for URL: "fbauth2:/" - error: "This app is not allowed to query for scheme fbauth2 was a symptom of the FBSDK attempting to open Facebook, which was not installed, so it was just a warning before it would fall back to browser-based FB auth.