First, is there even a prefix called "Mod Repository" defined in your wiki admin panel's Prefix Manager? If not, and if you don't want one, you must make sure that "[No Prefix]" is in the list of Allowed Prefixes for the area called "Mod Repository".
If the prefix exists, it is likely that the Prefix "Mod Repository" is not in the list of Allowed Prefixes for the area called "Mod Repository".
Whether it exists or not, please check your area settings. You probably need to add "Mod Repository" or "[No Prefix]" (depending on the above) to the list of Allowed Prefixes in this area, or you need to rename pages in the area so that they use a prefix that is allowed.