remove last comma in the file

If there's only one line in the file
sed -i .bak 's/\(.*\),/\1/' file.txt
if there are more lines in the file, do all the lines contain a comma?
does the last line in the file contain a comma?

file has only one line.
i just need to delete last comma.

then the above should work
if there are more lines, it will delete the last comma on every line,
(among them the last comma in the file, but may also include other commas)

when i try the sed command, its says

sed: Not a recognized flag: i
Usage:  sed [-n] Script [File ...]
        sed [-n] [-e Script] ... [-f Script_file] ... [File ...]

Command works perfectly.
if possible could you please explain the command  sed  's/\(.*\),/\1/'

Thanks