You will need to give more detail on what you are doing, but that is an arabic character set and probably doesn't have an 'M'.
What are the character sets for the database you are using? (select * from nls_database_parameters where parameter in 'NLS_CHARACTER_SET','NLS_NCHAR_CHARACTERSET'))
What is the datatype of the column whose value you are tryig to set?
I suspect this ins't really a SQL Developer problem. Does it also happen in SQL*Plus?
Are you trying to use the normal Latin letter M? Please, describe the steps you take as detailed as possible.
Latin letter 'M' can be encoded in any supported Oracle character set, including AR8MSWIN1256, so the message makes no sense. You may, however, try to use the identically looking Cyrillic or Greek letter, which cannot be encoded in AR8MSWIN1256, indeed.