Unexpected Output during activation

When I activate the plugin I get the following error:

The plugin generated 13449 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

I ran a debug and here’s what it says:

[body] => {“type”:”https://developer.mailchimp.com/documentation/mailchimp/guides/error-glossary/”,”title”:”Invalid Resource”,”status”:400,”detail”:”The resource submitted could not be validated. For field-specific details, see the ‘errors’ array.”,”instance”:”036268b1-6116-4753-9845-1abf36203975″,”errors”:[{“field”:”url”,”message”:”We couldn’t verify the URL is working. Please double check and try again. HTTP Code: 400″}]}
)
!
_________ERROR__________ !
Array
(
[error] => 400: The resource submitted could not be validated. For field-specific details, see the ‘errors’ array.

#0 /home/*****/public_html/wp-content/plugins/kc-s2mmc/kc-s2m-mc-free.php(465): KC_ErrLog(Array)
#1 /home/*****/public_html/wp-content/plugins/kc-s2mmc/kc-s2m-mc-free.php(200): kcs2mmc_listWebhookAdd(Object(KCMailChimpMailChimp), ’25ae7f8211′, ‘https://inte…’, Array, Array)
#2 /home/a*****/public_html/wp-content/plugins/kc-s2mmc/kc-s2m-mc-free.php(24): kc_s2m_mc_setWebhooks(Array)
#3 /home/*****/public_html/wp-includes/class-wp-hook.php(286): kc_s2m_mc_activate(false)
#4 /home/*****/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array)
#5 /home/*****/public_html/wp-includes/plugin.php(465): WP_Hook->do_action(Array)
#6 /home/*****/public_html/wp-admin/includes/plugin.php(660): do_action(‘activate_kc-s2m…’, false)
#7 /home/*****/public_html/wp-admin/plugins.php(44): activate_plugin(‘kc-s2mmc/kc-s2m…’, ‘https://inte…’, false)
#8 {main}