If you are not able to add a redirect, you may get a prompt that says the URL is reserved.
This occurs when part of the From URL is linked to a redirect that is already active on your website and cannot be changed. This is most often related to a feature (such as /calendar) or a hard-coded page (such as accessibility). These already have set destinations and cannot be used.
You may receive an error about a From URL not being able to add a redirect due to the risk of overwriting.
This means that the From URL already exists within the system. This can be under either All Sites or under a specific URL.