Though the question was asked many times, I can't find explanation for Nvidia RTX 2060. Nvidia drivers got OpenCL support. I don't understand why Blender says no GPU detected to render cycles with OpenCL. Did blender drop OpenCL support in favour of CUDA?
That is because Cycles in Blender 2.8x only uses OpenCL for AMD GPUs. For Nvidia GPUs you have to select either CUDA or OptiX (only for RTX cards until 2.90, then for Maxwell GPUs and higher). This is documented in Blender's manual.
CUDA and OptiX are supported for GPU rendering with Nvidia graphics cards.
OpenCL is supported for GPU rendering with AMD graphics cards.