Problem displaying site after transferring

Hi,
Many thanks for all of your help so far Rustaurius.
I’ve now completed my site after working on it in a sub-folder. I’ve moved it to the main public folder but the problem is that it displays without the ‘www’ prefix but not with it; The error message when you type in www. reads:

‘Warning: Cannot modify header information – headers already sent by (output started at /var/www/*****/public_html/wp-content/plugins/ultimate-product-catalogue/Functions/Install_UPCP.php:235) in /var/www/*****/public_html/wp-includes/pluggable.php on line 875

(Also, I can no longer log-in to my WP panel as the same error message as above appears)

I’ve contacted the host company about this & they suggested contacting yourself?

Thanks again

https://wordpress.org/plugins/ultimate-product-catalogue/