You cannot modify the properties of a file that's checked out

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This warning has the following cause and solution:

• You were attempting to change one of the properties of a file that's checked out from the command line.  This message is usually accompanied by an additional message informing you who has the file checked out.

  Identify who has the file checked out with the ss status <filename> command, then have them check in the file.  Now you can rerun the ss properties command.