I am able to scp files based on their extensions as, scp sk@localhost:/home/sk/*.{txt,text} .But when I try to scp a single extension type, it fails, scp sk@localhost:/home/sk/*.{txt} .I was able to solve this error by removing the flower brackets in case#2, I am just curious why does using flower brackets fail in case, there is only file extension type.
Braces don't work when there is a single element
shell;brace expansion
Assuming you're using bash, the documentation says:A correctly-formed brace expansion must contain unquoted opening and  closing braces, and at least one unquoted comma or a valid sequence  expression. Any incorrectly formed brace expansion is left unchanged.Thus, {foo} is not a correctly-formed brace expansion:$ bash -c 'echo {foo} {foo,bar}'{foo} foo bar