This week's Windows 10 Insider Preview Fast ring build, 14977, is available for Mobile only, so PC users are out of luck until next week. Unfortunately, some Windows phone users might be out of luck as well.
As it turns out, devices with 8GB of internal storage will likely throw an error with the code 0x800700b7. Jason Howard of the Windows Insider engineering team published a post on the Answers forum saying that there's also no workaround for this.
You'll have to wait until next week's Fast ring build, which will undoubtedly be disappointing considering that you've been waiting for three weeks for a build already. There won't be any long-term harm to your device for encountering this error; it will simply fail to update.
If you end up with an annoying notification to restart to install updates - despite it continually failing to install - you can always simply switch your phone to the Slow ring for now, as the Slow ring build is 14965, which is the same as your Fast ring build.
Sadly, there are quite a few lower end Windows phones out there that will likely be affected by this, such as the Lumia 535, 540, 550, 635, 640, 730, 735, and more. Everyone will just have to wait it out.