Are you talking about the index.php file? That should be in your public_html folder (the root folder) not in your templates folder. If you mean index.htm file, that shouldn't affect whether your site can access the index page. I was able to remove index.htm from my templates folder and the site was fine.
Check if you have an index.php page
Also if you're using an online host for your site and all the files are as they should be then it might be an issue with your host. I have a script hosted on GoDaddy and I had so much trouble setting it up. Everything was a fresh install and yet it couldn't find the index page and just said it didn't exist.
I ended up having to wait a few days, change the PHP version my site used multiple times, reset all the files, reinstall the script... and it finally worked.
Still not entirely sure why it didn't work the first few times
