Facts are:
Yes, IdFTPCommon.pas is present in the same directory as IdFTP.pas, that is
"/Developer/lazarus/components/indy-10.2.0.3/fpc/IdFTP.pas"
In IdFTP.pas, the uses section contains IdFTPCommon in the same letter case as the file IdFTPCommon.pas. I have even tried to add 'in /Developer/lazarus/components/indy-10.2.0.3/fpc/IdFTPCommon.pas' without success.
BUT, you may be right when you point out a letter case issue because error message is about IdFtpCommon, not IdFTPcommon...