OpenId requires the redirect URIs be set up prior. If, during an auth request, the platform sees one that isn't set up (i.e. it isn't currently set in the tool configuration), then it'll error like you've seen.
It's possible to have the tool url and redirect URI contain the same thing (in fact when using Moodle-to-Moodle LTI, this is the case, as is the case with some other tools too). This is considered normal.
It's also possible to have a redirect URI that is different to the tool url, and that's fine too (also normal). Depending on how the tool decides to handle different deployments, clients, etc, it may wish for the platform to redirect to something other than the tool/launch URL, and that's ok. As long as it's a registered before the launch. Here's an example tool.
Hope that helps,
Jake
It's possible to have the tool url and redirect URI contain the same thing (in fact when using Moodle-to-Moodle LTI, this is the case, as is the case with some other tools too). This is considered normal.
It's also possible to have a redirect URI that is different to the tool url, and that's fine too (also normal). Depending on how the tool decides to handle different deployments, clients, etc, it may wish for the platform to redirect to something other than the tool/launch URL, and that's ok. As long as it's a registered before the launch. Here's an example tool.
Hope that helps,
Jake