Could not load dynamic library 'libnvinfer.so.6'

Most of these messages are warnings, not errors. They just mean that libraries to use an Nvidia GPU are not installed, but you don't have to have any Nvidia GPU to use Tensorflow so you don't need these libraries. The comment by jakub tells how to turn off the warnings:

export TF_CPP_MIN_LOG_LEVEL="2"

However, I too run Tensorflow without Nvidia stuff and there is one more message that is an error, not a warning:

2020-04-10 10:04:13.365696: E tensorflow/stream_executor/cuda/cuda_driver.cc:351] failed call to cuInit: UNKNOWN ERROR (303)

It should be irrelevant because it too refers to cuda, which is for Nvidia. It doesn't seems to be a fatal error though.

I got this warning as a result of (accidental) update of libvnifer6 package. It got updated to 6.0.1-1+cuda10.2 while original installation used 6.0.1-1+cuda10.1.

After I uninstalled packages referencing cuda10.2 and re-ran

sudo apt-get install -y --no-install-recommends libnvinfer6=6.0.1-1+cuda10.1 \
    libnvinfer-dev=6.0.1-1+cuda10.1 \
    libnvinfer-plugin6=6.0.1-1+cuda10.1

this warning went away.

This is a warning, not an error. You can still use TensorFlow. The shared libraries libnvinfer and libnvinfer_plugin are optional and required only if you are using nvidia's TensorRT capabilities.

TensorFlow's installation instructions list the GPU dependencies:

The following NVIDIA® software must be installed on your system:

• NVIDIA® GPU drivers —CUDA 10.1 requires 418.x or higher.
• CUDA® Toolkit —TensorFlow supports CUDA 10.1 (TensorFlow >= 2.1.0)
• CUPTI ships with the CUDA Toolkit.
• cuDNN SDK (>= 7.6)
• (Optional) TensorRT 6.0 to improve latency and throughput for inference on some models.

You can install these on Ubuntu 18.04 with the following commands (taken from the TensorFlow documentation):

# Add NVIDIA package repositories
wget https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64/cuda-repo-ubuntu1804_10.1.243-1_amd64.deb
sudo dpkg -i cuda-repo-ubuntu1804_10.1.243-1_amd64.deb
sudo apt-key adv --fetch-keys https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64/7fa2af80.pub
sudo apt-get update
wget http://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64/nvidia-machine-learning-repo-ubuntu1804_1.0.0-1_amd64.deb
sudo apt install ./nvidia-machine-learning-repo-ubuntu1804_1.0.0-1_amd64.deb
sudo apt-get update

# Install NVIDIA driver
sudo apt-get install --no-install-recommends nvidia-driver-430
# Reboot. Check that GPUs are visible using the command: nvidia-smi

# Install development and runtime libraries (~4GB)
sudo apt-get install --no-install-recommends \
    cuda-10-1 \
    libcudnn7=7.6.4.38-1+cuda10.1  \
    libcudnn7-dev=7.6.4.38-1+cuda10.1


# Install TensorRT. Requires that libcudnn7 is installed above.
sudo apt-get install -y --no-install-recommends libnvinfer6=6.0.1-1+cuda10.1 \
    libnvinfer-dev=6.0.1-1+cuda10.1 \
    libnvinfer-plugin6=6.0.1-1+cuda10.1