Can you show us your linker command line?
Given the error message, you try to link a file called "b" instead of a prm file name in the command line. One possible cause could be a space between an option and its argument, or a command line with just a "b", missing double quotes if the path/filename contains spaces.
Possible command lines causing this:
b
a b
-oa b.elf test.prm
Corrected:
"b.prm"
"a b.prm"
-o"a b.elf" test.prm
Daniel