It would be helpful to know your computer information such as Windows version, AMD Driver version, Make & Model of Motherboard, CPU, and Ram memory.
First, download GPU-Z and make sure it shows OpenCL box checkmarked at the bottom left of the image for your RX5700 GPU Card.
*New GPU models with newer chip architecture may not be supported by the current software. New fahcores and/or new drivers with updated optimizations may be required for full support. New fahcores will not be available until some time after the new hardware products are sold to the public. Then the fahcores and drivers may be updated, tested, and verified before released for folding.
Note: Under Windows, antivirus software can interfere with the Folding@home client files and cause errors. We suggest configuring antivirus software to exclude the FAH client directory and especially the Work directory from the antivirus scanning list. This can be done by going into the exclusion list panel that every antivirus should have. The work subdirectory contains semi-random binary data and can confuse overly aggressive heuristic virus scanning.
Note: Installing OEM specific GPU drivers can be problematic in Linux. Please review the AMD GPU driver installation instructions for Linux. Please review the NVidia GPU driver installation instructions for Linux. See also this Folding Forum topic for tips installing the drivers.
It is possible as mentioned in the FAQ that the latest AMD Drivers may not be supported yet by the current software.
Here is the latest RX5700XT Driver: AMD Radeon™ RX 5700 XT Drivers & Support | AMD
You can also install a previous older AMD Driver and see if OpenCL works with the current software: AMD Radeon™ RX 5700 XT Previous Drivers | AMD
OpenCL is detected but running FAH simulations gives errors with AMD 5700 XT and latest drivers on Windows and Linux.