Mixed content error message

I am seeing an endless loading spinner on the checkout page on my site since installing the MailChimp for Woocommerce plugin. I can see the following error when looking at the console:

Mixed Content: The page at ‘https://www.lbm-art.com/checkout/’ was loaded over a secure connection, but contains a form that targets an insecure endpoint ‘https://lbm-art.us12.list-manage.com/subscribe/post?u=e6debbf267cc0e6a553066afd&id=f296280634’. This endpoint should be made available over a secure connection.

I am on a free MailChimp account so I have no idea how to make the link to the list secure or if there’s another way to rectify this. Can anyone help please? Thanks in advance.

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