Can you please confirm you are using the most recent Yoast v7.1? You can learn more about updating here: https://kb.yoast.com/kb/how-can-i-update-my-premium-plugin/
Can you confirm you are using WordPress 4.9.4? You can check by clicking on the W in the top left and selecting About. If you need to update, please check with your host provider. Please know that if you are using an older version of WordPress Core you may experience unexpected behavior with Yoast. This guide explains more:
https://yoast.com/why-we-dont-support-old-wordpress-versions/
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?If you update to the most current version of WordPress, does this resolve the issue?
