User839733648 posted
Hi Kwe7777,
According to description and codes, I'm sorry that I could not reprodcue your issue successfully.
When I run your code, there is only one error message after selecting an image on my computer.
Uncaught ReferenceError: previewFile is not defined
And I've checked your js file, there is no previewFile function. So the error exists.
I've tried to remove onchange="previewFile()" and run the code, it works well.
I'm curious that I do not see the CSS file error on my side. Have you shared the complete code?
Or I suggest that maybe you could download the github's project and run again to test it.
Here is my running result.

Best Regards,
Jenifer