errorslyx
I received a LyX file from my friend and when I tried to compile it, LyX showed me this these errors.
I have no idea what is going on. Hope you can help me out, thanks a lot.
Update: Here is the minimal example and I'm using LyX 2.1.3, thanks for your help.
https://goo.gl/JTKzXV
I would say your editor has exchanged the unicode chars by this \UTF-commands when saving the file. A search in the TeXshop forum came up with this advice: "Please UNCHECK TeXShop->Preferences->Misc->pTeX support-> utf export".
As egreg mentions in a comment, the reason you get the error is that neither the amsfonts nor amssymb package, where \mathbb is defined, is loaded. To force LyX to add one of these you can do one of two things:
Go to Document --> Settings --> LaTeX preamble and add there either
\usepackage{amssymb}
or
\usepackage{amsfonts}
Go to Document --> Settings --> Math options. Uncheck Use AMS math package automatically and check Use AMS math package, as in the screenshot below. LyX will now have added
\usepackage{amsmath}
\usepackage{amssymb}
to the document preamble.
The first of these, the automatic one, adds these packages only if you use some of their features through the LyX GUI. E.g. if you do Insert --> Math --> AMS Align, the amsmath package will be added, and if you e.g. add some text with Blackboard bold via the math font button on the toolbar -- 
-- then amssymb will be added.
Best Answer
This seems to be a bug in LyX. LyX is supposed to automatically load the right packages (unless you use ERT, then you're on your own). For some reason, when the document is set to Vietnamese, the package is not loaded automatically. To work around this bug, go to Document > Settings > LaTeX Preamble and put the following: