Symbol's function definition is void: mathpix-app-id

[Hongyi Zhao <hongyi.zhao@gmail.com>]

I try to test the mathpix.el package located at
<https://github.com/jethrokuan/mathpix.el>, using Emacs git master
version on Ubuntu 20.04 with the following methods:

# Clone the github repo:
$ cd .emacs.d/elpa
$ git clone https://github.com/jethrokuan/mathpix.el.git mathpix.el.git

Add the following code snippet into ~/.emacs.d/init.el:

(load-file "~/.emacs.d/elpa/mathpix.el.git/mathpix.el")
(mathpix-app-id "app_id")
(mathpix-app-key "app_key")

Start Emacs and run the following command to do the test:

"M-x mathpix-screenshot RET"

Then I meet the following error:

Warning (initialization): An error occurred while loading
‘/home/apollo/.emacs.d/init.el’:

Symbol's function definition is void: mathpix-app-id

To ensure normal operation, you should investigate and remove the
cause of the error in your initialization file.  Start Emacs with
the ‘--debug-init’ option to view a complete error backtrace. Disable
showing Disable logging

Any hints for fixing this problem?

Regards
-- 
Assoc. Prof. Hongyi Zhao <hongyi.zhao@gmail.com>
Theory and Simulation of Materials
Hebei Vocational University of Technology and Engineering
NO. 552 North Gangtie Road, Xingtai, China