Hi @Megan,
Thank you for reaching out to us.
Basically this type of issue is either related to theme or plugin conflict. May I know if you have checked marketplace plugin with WooCommerce and standard theme (storefront/twenty). Please deactivate all your plugin, except WooCommerce and WCMp + addon. You can then check the flow. If everything is working fine, in order to detect which plugin is causing conflict- please activate each plugin consecutively and check the flow.
Similarly, for theme conflict- please deactivate the current theme and check with standard WooCommerce theme (storefront/twenty). It is suggested to please perform these tests on your staging website and keep us posted with the outcome.
Awaiting your response.