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.