If you are not able to add a redirect, you may get a prompt stating the URL is reserved.
This is due to a part of the From URL being linked with a redirect already active on your site that cannot be changed. This is most often related to a module (such as /calendar) or hard-coded page (such as accessibility). These already have set destinations and cannot be used.
If you 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.