The warning is telling you that the code has mixed formatting, such as some lines having spaces for space and others having tab for space. You can fix each line individual or use an automated tool to reformat it for you.
I use Notepad++ with the TextFX plugin installed. That allows me to click a button to have it reformat the code for me. It works very well.
Explanation of the process:
https://qnaplus.com/how-to-re-indent...using-notepad/
Link to Notepad++:
https://notepad-plus-plus.org/
I hope this helps...