RSVPMaker has been temporarily removed from the wordpress.org repository because of a SQL Injection vulnerability in some old code. I have provided a patch, which the powers that be are reviewing. Meanwhile, I've made it available here.
I apologize for this potentially serious error.
Updates should be available through the plugin repository again soon.
If you're running WordPress 5.5, it's now possible to update a plugin using a ZIP file. The system will tell you there is already an existing plugin directory for RSVPMaker then prompt you to confirm that you want to update to the uploaded version.