Fundraising Error When Making Donation

Hi,

I'm getting this error when trying to submit a donation;

Warning: Cannot modify header information - headers already sent by (output started at /home/iamclair/public_html/wp-content/plugins/wpmu-dev-seo/wds-files/wds-onpage/wds-onpage.php:64) in /home/iamclair/public_html/wp-includes/pluggable.php on line 866

Any advice? Thanks,

Claire