Port 80 is already in use by the snap package. Port 80 is default http so you will have to change this to another port to setup the other service to listen on 80. However you will have to port forward from port 80 to the new port you bind your Nextcloud SNAP or use DNS hook for the Lets encrypt or the certbot auto renewal will not work as it uses port 80 AND port 443. Alternative is a reverse proxy for proxying to the different services based on hostname.
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