Your function doesn't return the correct result. It adds an extra space at the end.

"The quick brown fox jumps over the lazy dog.": expected 'ehT kciuq nworb xof spmuj revo eht yzal .god ' to equal 'ehT kciuq nworb xof spmuj revo eht yzal .god'

Even if you fix that, your solution also does not handle words with double spaces correctly either (as in the 4th sample test).

You checked it wrong:

Input:   "The quick brown fox jumps over the lazy dog.": 
expected 'ehT kciuq nworb xof spmuj revo eht yzal .god ' 
to equal 'ehT kciuq nworb xof spmuj revo eht yzal .god'
                                                      ^ here your solution is different 

You need to tell us what language you tried, and show us your code. Otherwise, we do not know what to fix.