I have installed Cuda 10.1 and cudnn on Ubuntu 18.04 and it seems to be installed properly as type nvcc and nvidia-smi, I get proper response:
user:~$ nvcc -V
I had the same issue. I fixed it by adding the below command to the '.bashrc' file.
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/cuda-10.0/lib64/
System configuration:
Ubuntu 16.04 LTS Tensorflow GPU 2.0beta1 Cuda 10.0 cuDNN 7.6.0 for Cuda 10.0
I used conda to configure my system.