WordPress theme index.php not working because it’s the WordPress file system does not match, here is the solution, it’s just a simple thing to do.

WordPress used the file structure hierarchy to make more effective the files and content.

so

In WordPress first important and required file on the theme

• header.php
• index.php
• sidebar.php
• footer.php

and If you are used to more files like

• archive.php
• single.php
• header.php
• footer.php
• page.php
• comment.php
• sidebar.php
• 404.php
• search.php
• searchform.php

WordPress index page not working

The WordPress file system works differently

If you have page.php then your WordPress always looks for page.php instead of index.php.

If you want to work on index.php then don’t use page.php or what you want to use in index.php that content include in page.php

that’s it.

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useful content from WordPress https://developer.wordpress.org/themes/getting-started/your-first-theme/

File system https://developer.wordpress.org/themes/basics/template-hierarchy/