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If the Beatmap Submission System (BSS) won't let you submit your own beatmap because it says that you do not own the map then do the following:
Go to the beatmap's folder.
You can get there easily by going to
File then
Open Song Folder in the
beatmap editor.
Exit osu! (This is to make sure that the changes are applied properly.)
Open the .osu difficulty files with any plain text editor (e.g. Notepad).
Enter your username for the Creator. (If your name has spaces, then use spaces.)
Set the BeatmapID to 0.
Set the BeatmapSetID to -1.
Make sure the beatmap folder name does not start with a string of numbers.
Delete the file(s) in the SubmissionCache folder. This is located in the osu! installation folder.
Open osu! again and try to upload your beatmap.
If you are still having issues, follow these steps:
Export the beatmap using the beatmap editor.
Exit the beatmap editor and then delete the beatmap in osu!.
Exit osu!
Go to the exported beatmap and change the extension from .osz to .zip (you will need file extensions to be visible from the windows folder options to do this).
Extract this .zip file.
Open the .osu difficulty files with any plain text editor (e.g. Notepad).
Enter anything as username for the Creator, but do not use your username.
Set the BeatmapID to 0.
Set the BeatmapSetID to -1.
Once you are done, zip the files back into the .zip file.
Rename this .zip file back to .osz.
Import it into osu! and try uploading it again.
If this fails, you may want to create a new topic thread in the
Help subforums to seek further help.