“Site Stats” page

on the Jetpack Sitestats page, and no where else, I get this message

Warning: Cannot modify header information – headers already sent by (output started at /home/outofpos/public_html/wp-admin/includes/template.php:1969) in /home/outofpos/public_html/wp-includes/rest-api.php on line 359

I have unplugged recent plug ins, which has helped in the past, but not sure what do here.