As you correctly surmised, the culprit is your obsolete php 5.6.

You need to remove the ":string" from that line:
Code:
function recaptcha_get_html($fieldset = false, $theme = 'light', $size = 'normal', $style = null): string {
to
Code:
function recaptcha_get_html($fieldset = false, $theme = 'light', $size = 'normal', $style = null) {