The code shown in the image and the compiler complaint in the sandbox result do not match. Could indicate that you made changes to the source code, but did not save before trying check50. Actually, one should compile (make) and test (./water) the program before ever submitting to check50.
The main problem is because you didn't save your work. I had the same problem because I was testing and didn't save last changes. Also, consider to change your b for 12. It's redundancy to use in your case.