The long and short of it is that the playlist is, for whatever reason, pointing to files that no longer exist in the place it expects them to be.
You said in your original post that you imported your playlist. What format was the playlist in on your drive before you imported it? If it was in a text-editable type file (m3u for example), you can open the original playlist in a text editor and look at it.
If the file names are like "File1.mp3" "Another Song.mp3", etc with no other information, that is a playlist that was created with relative paths (the program creating it expects the program playing it - usually itself - to already know where the files are).
If the file names are like "D:\Music\File1.mp3" C:\AlbumName\Another Song.mp3" etc, those are absolute paths. Those are the only kinds of playlists that can reliably be moved from one program into another on the same computer. No matter what the defaults are for the particular program, it will know exactly where to find each file.
If they're relative, you have a problem I don't know how to fix besides recreating the playlist from scratch.
If they're absolute and you haven't rearranged / renamed the files in any way, you can search/replace the wrong path to the right path in your text editor in the source playlist. Search C:\OldComputerMusic and replace with C:\NewComputerMusic or whatever the old and new folder structures are.., save the playlist, and re-import the corrected version to MusicBee.
If you've already moved your files around from the way they were arranged on the old computer or renamed them, you're probably screwed.
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