that error happens becauzse of output on the page before the header function.
the error is saying the proble is around line 541 of the tienda.php page.
output started at /home/greenpe/public_html/Tienda/tienda.php:541
if you need further help finding the problem, attach a copy of that page.
NOTE: please use a descriptive title for your posts, don't include the entire message in the tittle filed, that is what the message field is for.