xavierblue on "Cannot modify header information"

I'm at my wits end here, I have a site that I'm trying to create. Now the site is fairly large and ran into the "Waing: Cannot modify header information - headers already sent by " error. Once I hit this error I found lots of help mostly pointing to white space issues and after a lot of searching my files for this white space I decided to experiment. I backed my files up and deleted and reinstalled the entire WP applications from scratch. After doing that I edited the main "page.php" so that it now says "get_template_part( 'template-parts/content', 'emailhistory' );"

The content-emailhistory.php file looks like this:

<form action="">
<input id="send" name="submit" type="submit" value="Edit Email"><?php include 'EditEmailLandingPage.php' ?></form>
</form>

the EditEmailLandingPage.php looks like this:

<?php
$submitted = "true";
wp_safe_redirect( 'thanks.html', 301 );
exit;
?>

I'm still getting the same error but with different files:

Waing: Cannot modify header information - headers already sent by(output started at /xxx/xxx/xxx/xxx/xxx/xxx/header.php:13) in /xxx/xxx/xxx/xxx/pluggable.php on line 1171

I've spent days trying to figure this out. I'm trying to implement the PRG standard. Is there something I'm doing wrong here?