GitHub SSH action failing: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain

Question

I have recently rented a VPS running CentOS 8. I want to make countinuous deployment from one of my repos, so that when I push to main the repo is updated. I followed this tutorial on a fresh CentOS installation, with the following action (this is my .yml):

name: Deploy
on: [push]
jobs:
  deploy:
    if: github.ref == 'refs/heads/main'
    runs-on: [ubuntu-latest]
    steps:
      - uses: actions/checkout@v1
      - name: Copy repository contents via scp
        uses: appleboy/scp-action@master
        env:
          HOST: ${{ secrets.HOST }}
          USERNAME: ${{ secrets.USERNAME }}
          PORT: ${{ secrets.PORT }}
          KEY: ${{ secrets.SSH_KEY }}
        with:
          source: "."
          target: "/var/www/deploy"

However, I get the following error:

Copy repository contents via scp

... 2020/12/02 18:05:57 error copy file to dest: ***, error message: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain drone-scp error: error copy file to dest: ***, error message: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain

I have attempted to ssh-add <private_key>, however I get the same error.