git checkout master error: the following untracked working tree files would be overwritten by checkout

Try git checkout -f master.

-f or --force

Source: https://www.kernel.org/pub/software/scm/git/docs/git-checkout.html

When switching branches, proceed even if the index or the working tree differs from HEAD. This is used to throw away local changes.

When checking out paths from the index, do not fail upon unmerged entries; instead, unmerged entries are ignored.

With Git 2.23 (August 2019), that would be, using git switch -f:

git switch -f master

That avoids the confusion with git checkout (which deals with files or branches).

And that will proceeds, even if the index or the working tree differs from HEAD.
Both the index and working tree are restored to match the switching target.
If --recurse-submodules is specified, submodule content is also restored to match the switching target.
This is used to throw away local changes.