Please use markdown to post your code next time ;-)

• you return a string literal if n == 0: you need to return a malloc()'ed string in each case. This is why the tests crash: they try to free() your returned string, but a string literal cannot be freed.

Other mistakes:

• your loop counter i starts at 1, but you print i + 1 to the string, so 1 sheep... is missing
• your memory allocation is not very robust: if n == 10000, "10000 sheeep..." will be 15 characters long, but you allocate only 13 for mur and 13 * n for murmur ---> for large n you will run out of memory and write out-of-bounds

Please mark your post as having spoiler content next time.

See the example in the description, the input is not in the error message, the first value is your wrong answer, the one with less spaces in it and the second one is the expected answer, with double spaces.
This is the test:

#                                     input value          expected reversed value
test.assert_equals(reverse_words('double  spaced  words'), 'elbuod  decaps  sdrow')

I am sorry but the text is not being reversed; what i can see is only the addition of a space between the words

'elbuod decaps sdrow' = > 'elbuod decaps sdrow' (only the spaces added here)

look at the length and the individual bytes of your string - it does not work perfectly, you are not measuring correctly. granted, I can see how that would be confusing, but, it also seems like the sort of thing to watch out for in C with all the footguns available

The error message is:

'elbuod decaps sdrow'     should equal 
'elbuod  decaps  sdrow'

Your solution does not preserve adjacent spaces.

oh , that's really seem to be my bad , im sorry

Backup your code, click reset, and paste it again.

but why the default testcases are wrong ?