"Warning - You only have to install a Great App to get your phone fully functioned." Keeps popping up It looks like this same thing happened to a user 2 months ago, but I can not reply to that question: Suspicious popup and &#39;Yellow Booster&#39; installation.  This thing goes away for hours, so we don't ever really know if it's gone, and then it pops back up.  Was this ever solved?  If anyone has found a solution, we would appreciate it.

On a different Droid forum, a lot of us started getting this problem recently. I have noticed a lot of us are on Verizon and on fairly new phones.  Personally, I am on a HTC 10, but other users are getting this pop up on Samsung and LG phones as well.  If anyone has anything that could help, I would greatly appreciate it!
I went through this same problem. 
Here is the solution:


First boot to safe mode:


Press the power button. A Power Off popup will show. Long press the Power Off popup. It will ask if you if you want to boot in safe mode. Press yes.

After starting in safe mode, goto Settings > Apps and scroll down to the bottom to find Yellow Booster App. On clicking, you will see a screen with details of the App. Press Uninstall and you will be able to successfully uninstall. Note that, while you can reach this screen normally from setting, you can't press uninstall until you are in safe mode.
As always, take effort to find other apps that are suspicious and remove them.
Yellow booster is also known to sometimes have Device Administrator privileges. If it added itself as one, remove it as well. You can't remove that in non-safe mode.