DevDrake

on Orangepi One you have to add a file like /etc/udev/rules.d/99-gpio.rules
SUBSYSTEM=="gpio*", PROGRAM="/bin/sh -c '\ chown -R root:gpio /sys/class/gpio && chmod -R 770 /sys/class/gpio;\ chown -R root:gpio /sys/devices/platform/sunxi-pinctrl/gpio && chmod -R 770 /sys/devices/platform/sunxi-pinctrl/gpio;\ '" add a group gpio and add the user to it, then you have rootless access to gpio

The question remains why PA20 cannot be used?
# echo 20 > /sys/class/gpio/export -bash: echo: Schreibfehler: Das Gerät oder die Ressource ist belegt. # cat /sys/kernel/debug/gpio gpiochip1: GPIOs 0-223, parent: platform/1c20800.pinctrl, 1c20800.pinctrl: gpio-10 ( |sysfs ) out hi gpio-20 ( |orangepi:red:status ) out lo I should take the time to test the other ways to access the GPIO ...

MS did not release deb/rpm packages for armhf/arm64, instead, you should manually install them, including dependencies. The drawback is the every time the runtime updates, you need to follow these steps again.
 
If you only need to run your .NET Core apps but not building them on the board, you can download arm64 runtime at https://dotnetcli.blob.core.windows.net/dotnet/Runtime/2.1.3/dotnet-runtime-2.1.3-linux-arm64.tar.gz and follow the steps here to install.
If you have a really powerful board (like one of those RK3399 boards) and want to build apps on it, use https://dotnetcli.blob.core.windows.net/dotnet/Sdk/2.1.401/dotnet-sdk-2.1.401-linux-arm64.tar.gz instead.
 
 
 
 

Did the SDK generate a directory like "bin/Release/netcoreapp2.1/linux-arm64/publish", or you are copying files from elsewhere?

I run my .NET Core apps on Armbian every day, but I've never installed any runtime separately. Instead, I install the dependencies, then use the .NET Core SDK to publish them on a x86 machine with 
dotnet publish -c Release -f netcoreapp2.0 -r linux-arm then copy the published files to the board and run it.
 
If you want to run a .NET Core app on Armbian ARM64, you should install every dependency with ":armhf" added. (e.g. apt install libunwind8:armhf)
Or, you could try .NET Core 2.1 with
dotnet publish -c Release -f netcoreapp2.1 -r linux-arm64 to generate ARM64 binary.
 
Beware that there's an issue which causes the .NET Core 2.1 ARMv7 runtime to crash after running for a while, so don't publish with netcoreapp2.1 linux-arm until the issue is closed, or your app will be very unstable.