The CUDA VERSION display within nvidia-smi was not added until driver 410.72
Your driver (390.87) doesn’t include this display.
Also, be aware that the CUDA VERSION displayed by nvidia-smi associated with newer drivers is the DRIVER API COMPATIBILITY VERSION. It does not indicate anything at all about what CUDA version is actually installed. A 410.72 driver will display CUDA VERSION 10.0 even when no CUDA toolkit is installed.