modprobe: ERROR: could not insert 'nvidia_352': Required key not available

my computer is ubuntu 14.04, when I try the sample in the /usr/local/cuda-7.5/samples ,I encountered the problem.
the specific log file as follows:

nbrobot@nbrobot:/usr/local/cuda-7.5/samples/bin/x86_64/linux/release$ ./clock
CUDA Clock sample
modprobe: ERROR: could not insert ‘nvidia_352’: Required key not available
CUDA error at …/…/common/inc/helper_cuda.h:1111 code=38(cudaErrorNoDevice) “cudaGetDeviceCount(&device_count)”

The GPU driver is not installed correctly.

Follow the instructions in the linux install guide carefully.

when I reboot my computer, the error is missing, but there is a new error.
my GPU is geforce 940mx, and it is not in the official list: https://developer.nvidia.com/cuda-gpus

nbrobot@nbrobot:~/NVIDIA_CUDA-7.5_Samples/bin/x86_64/linux/release$ ./deviceQuery
./deviceQuery Starting…

CUDA Device Query (Runtime API) version (CUDART static linking)

modprobe: FATAL: Module nvidia not found.
cudaGetDeviceCount returned 38
-> no CUDA-capable device is detected
Result = FAIL