For some reason the osrs client is claiming my account doesn't exist is anyone else having the same problem?
Not just you but for everyone, the old OSRS client doesn't exist anymore. We are all migrated to the new Jagex Accounts platform since November 7, 2023.:
"The benefits of Jagex accounts are inherently linked with the Jagex Launcher - both are interconnected projects in our mission to improve security and the player experience for our games. As part of our continued rollout of these new systems, from November 7th, players will require a Jagex Account to access the Jagex Launcher.
Once this goes live, if you have an existing RuneScape account logged in to the launcher, you will not be impacted by this until you next log out of the Launcher. If you do log out, you will need to upgrade to a Jagex Account to continue using the Jagex Launcher."