Can't make new dir with mkdir

Probably a parent directory in the path does not exist.

You can try with

mkdir -p /path-to-directory/directory-name

See man mkdir

   -p, --parents
          no error if existing, make parent directories as needed

If you get a permission denied error, you have not permissions to create a directory in the specified path.

Check if you can get around the problem by modifying the group membership or ownership, so that you get the permission needed for the whole directory path involved.

Otherwise you need elevated permissions, so try with sudo

sudo mkdir -p /path-to-directory/directory-name

sudodus's answer appropriately addresses how to create all directories along the given path. Alternative way would be via Python. This is especially useful if you're developing software for Ubuntu in Python and need such functionality. Calling mkdir as external command would add overhead of additional process and extra forking which would waste resources. Luckily Python's standard library, specifically os module has makedirs() function:

$ python3 -c 'import os,sys;os.makedirs(sys.argv[1])' test_1/test2/test_3
$ tree test_1
test_1
└── test2
    └── test_3

2 directories, 0 files

Note that such behavior also can be achieved in Perl, which is another scripting language that comes by default with Ubuntu.

I had this when the current directory literally didn't exist anymore.

I was in directory temp:

mark@mark:~/PycharmProjects/temp$ mkdir foo
mkdir: cannot create directory ‘foo’: No such file or directory

I saw the light when the current directory was empty (not even the hidden . and .. existed):

mark@mark:~/PycharmProjects/temp$ ll
total 0

One directory up temp does exist, but it is another directory with the same name. PyCharm must have deleted and recreated the project directory when I was rolling back too much changes and undoing the rollback.

mark@mark:~/PycharmProjects/temp$ cd ..
mark@mark:~/PycharmProjects$ ll
total 12
drwxrwxr-x  3 mark mark 4096 Nov  2 14:26 ./
drwxr-xr-x 40 mark mark 4096 Nov  2 14:50 ../
drwxrwxr-x  3 mark mark 4096 Nov  2 14:42 temp/
mark@mark:~/PycharmProjects$ cd temp
mark@mark:~/PycharmProjects$ mkdir foo
mark@mark:~/PycharmProjects$