The site was likely using features of the Pro version and cannot be replaced by the free version. You need to reactivate the Pro version to correct site issues.
When you get the error that the update package is not available this means that it is not registered. This can sometimes be corrected by going to the updates activation page and deactivating the license and then re-activating it.
If this does not work then you can log in to your ACF account and download the new version of Pro from there. You can delete the old one and install the new version.
