If you are not able to add a redirect, you may get a prompt that says The Link From URL path is reserved. Please enter a different URL.
This is due to a part of the From URL that links with a redirect already active on your site that cannot be changed. This is most often related to a module (such as /calendar) or a hard-coded page (such as accessibility). These already have set destinations and are not available for use.
You may receive an error about a From URL not being able to add a redirect due to the risk of overwriting. It will say The From URL is not allowed because it will overwrite an existing URL.
This means that the From URL already exists within the system. This can be under either All Sites or under a specific URL.