What level are you trying to load, where did you get the level, and where did you put the interiors?
Some levels (esp. older ones) use CLA interiors for its interiors. But because you have the map files, the level must not be from level packs/CLA. Make sure to put the interiors in the interiors folder (the maps aren't needed), as well as putting interiors in folders inside the corresponding folders in the interiors folder.
As opposed to missing interiors, the reference to the interior may be incorrect. If you can, open up the .mis file for the level in a text editor, and for each of the places it says /interiors, make sure that the path is not misspelled, and that the interior is in the right place. Sometimes, the interior may not be in the interiors folder, but instead, may be in a folder called interiors_mbp, or similarly named, and so the level may not work. When the interiors are located and in the right place, reopen the game, and hopefully it will work.
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