The current code is incomplete since I don’t see a project or cmake_minimum_required version, in addition the error you are seeing is due to the variable CMAKE_CUDA_ARCHITECTURES being set to an empty string, so somewhere before this code block that must be happening.
You most likely have something like
set(CUDA_ARCHS "60")
set(CMAKE_CUDA_ARCHITECTURES "${CUDA_AARCHS}") #typo
That makes the CMAKE_CUDA_ARCHITECTURES variable an empty string.
Note:
Usage of FindCUDA and the associated cuda_add_executable, and cuda_add_library calls are deprecated and note supported. Please switch over to the CUDA language support