Hreflang conflicts within page source code

Hello,
I hope you’re doing well. I’m encountering an issue flagged by a Semrush audit: “hreflang conflicts within the page source code.” Since I use Polylang on my website, the hreflang tags are automatically generated, but it seems there’s no x-default tag included.

I believe adding the x-default tag manually might resolve the issue, but I’ve tried both adding it via functions.php and directly in header.php without success. Could you please advise if there’s a way to set the x-default hreflang tag through the Polylang plugin or guide me on the best way to resolve this conflict?

Thank you for your help!

Best regards,
George