Error: HTTP 422 Unprocessable Entity when inviting a user or processing a subscription

This error can during the manual invite process of a user or during the checkout process when a user selects a plan and goes through the checkout flow. The error happens because Plex could not add a user to your Plex server. Here are some of the reason why that may happen:

• Your libraries are out of date. If you recently made changes to your Plex servers or libraries and they did not sync to Streams Manager, the error will occur because invitation will reference a library that does not exist or has changed. Run a server sync on Servers panel to resolve the issue.

• Ensure that your server machine ID did not change. See this article.

• Your server is full. If your server has 99 users (including fiends and managed/home users), no more users can be added.

• You are trying to invite a user that is already in your Plex home.

If the error occurred during the checkout process of a recurring PayPal subscription, you will have to resolve the issue manually as PayPal subscription will likely have succeeded but user was not given Plex access.