You tried to connect to your own Nukkit server running on Pteradactyl. However, it failed quickly and you got this message: "logged out due to Outdated server." You look in the logs for more information, and that's all the logs show, also: "logged out due to Outdated server." So you search online to see if there are instructions for how to fix this. Nothing. Nada. Maybe that's how you get to this page? If so, you're in luck, the fix is pretty straightforward.
Your client is at the latest version, but the server is no longer the latest. How do you get the latest server? You compile it yourself, and then upload it.
The following instructions are at https://github.com/CloudburstMC/Nukkit:
Build JAR file:
git clone https://github.com/CloudburstMC/Nukkit
cd Nukkit
git submodule update --init
chmod +x mvnw
./mvnw clean package
The compiled JAR can be found in the target/ directory.
After that, copy the target/nukkit-XYZ.jar to "server.jar"
cp target/nukkit-1.0-SNAPSHOT.jar server.jar
Within Pterodactyl, at the Console, stop the server so you can replace it.
Then use the File Manager to upload the new file, overwriting the current server.jar file. You can archive the existing one if you want to have it around for any reason. It's the only file you need to change, all the rest will be fine as they are.
Start the server and you'll be at the latest version.
Update August 2022:
You can skip the compiling above by downloading the latest binary from the following URL: