Here is the solution:
Open the plugin code and move to wp-seo-structured-data-schema/lib/classes/KcSeoOutput.php
Find this code:
“telephone” => ! empty( $settings[‘contact’][‘telephone’] ) ? $KcSeoWPSchema->sanitizeOutPut( $settings[‘contact’][‘telephone’] ) : ”,
Change it into
“telephone” => ! empty( $settings[‘contact’][‘telephone’] ) ? $KcSeoWPSchema->sanitizeOutPut( $settings[‘contact’][‘telephone’] ) : $settings[‘site_telephone’],
We need give the value if the variable is empty. It seems the scheme code can’t accept empty phone number.
