While attempting to install build 14977 for Mobile, you may hit an error: 8007007B
Unfortunately if you hit this error, you will not be able to install 14977. There is no workaround. You'll need to wait for our next Mobile build release.
This issue does not affect all devices and there is no long-term harm from encountering it. The device will simply fail the install and boot back to your current OS build.
We're already working on a resolution so all applicable devices may receive the next update.
Thank you as always,