Giorgia,

I ran into the same error and found a way around it. I am getting another error down the line now but I figured I'd share this in case it helps. Here's what I did, starting from the beginning of the install process (I also added a command to create the DTIPrep-build/bin directory because for some reason the installation wasn't creating it correctly):
1) git clone https://github.com/NIRALUser/DTIPrep.git
2) mkdir DTIPrep-build; mkdir DTIPrep-build/bin; cd DTIPrep-build
3) cmake ../DTIPrep
4) (Open the file ../DTIPrep/SuperBuild/External_ITKv4.cmake with a text editor.)
5) (Change the default value of "-DITK_LEGACY_REMOVE" to "OFF".)
6) make

Here's the next error I'm getting that I haven't been able to solve:

CMake Error at /usr/local/share/cmake-3.12/Modules/ExternalProject.cmake:2425 (message):
URL_HASH is set to

SHA512="1ee2c7bec3657f6846e63c6dfa71410563830d2b951966bf0123bd8f4f2f5d6b50f13b76d9a7b0eae70e44856f829ca6ceb3d080bb016
49d1572c9f3f68e8eb1"
but must be ALGO=value where ALGO is
MD5|SHA1|SHA224|SHA256|SHA384|SHA512|SHA3_224|SHA3_256|SHA3_384|SHA3_512
and value is a hex string.
Call Stack (most recent call first):
/usr/local/share/cmake-3.12/Modules/ExternalProject.cmake:3100 (_ep_add_download_command)
CMake/itkExternal_FFTW.cmake:118 (ExternalProject_add)
CMakeLists.txt:321 (include)

Thanks,
-John