I can't find any working information, how to fix this, and before I was trying to install some other drivers for my Vega, but i accidentally broke my previous installation.
You most probably have installed the open source driver stack that comes with Mint.
On any GNU/Linux distribution, Blender only supports the official AMD proprietary drivers for rendering with OpenCL, meaning you will need to install one of the following AMD OpenCL drivers:
For Linux Mint, this might be a good start
There might be another method to just install the OpenCL part of the AMMDGPU-PRO driver, which is described here, but I don't have the means to test it.