I have the same issue. I signed back into my account from Preferences -> Account and that was the original cause of the error. But I can't seem to find a way to tell Xcode that it should make the reattempt to download symbols now. Any idea how to delete the cache or do I just have to wait 1 - 24 hours for this to fix itself? HumanReadableNSError: The run destination MARKS IPHONE PRO is not valid for Running the scheme 'Set Status WatchKit App'. The device “Mark’s Apple Watch” is not available because there was an error downloading its symbols. Please see the underlying error message for more information. IDEFoundationErrorDomain (1): ================================== |  HumanReadableNSError: The device “Mark’s Apple Watch” is not available because there was an error downloading its symbols. |   |  Please see the underlying error message for more information. |   |  com.apple.platform.iphoneos (-1): |   |  ================================== |   |  |  HumanReadableNSError: Mark’s Apple Watch via MARKS IPHONE PRO: Failed with HTTP status 403: forbidden |  |   |  |  DVTACSURLSessionDelegate (1): |  |  ==DVTDeviceErrorIsProxied: 1 I just need to know how to flush the cache for this. Is there a way to do that or do I need to delete Xcode and install it all over again?

The solution for me was to (After being stuck at the Black Apple Screen with a maybe 2% full loading bar for over 12 hours) hard restart with the power button. From that point forward the install process took all of 5 minutes. That is on a Mac mini (Late 2014) - 1.4 GHz Dual-Core Intel Core i5 - 4 GB 1600 MHz DDR3.

stuck on the “Black background, white apple logo & progress bar” now for about 4 hours with no discernible progress bar movement. That's where I am too. It's been probably about 5 hours now.