What about now?

.drone.yml

@@ -26,7 +26,7 @@ steps:
   - name: deploy
     image: ubuntu:20.04
     environment:
-      SSH_PRIVATE_KEY:
+      SSH_DRONE_PRIVATE_KEY:
         from_secret: ssh_private_key
       SSH_USERNAME:
         from_secret: ssh_username
@@ -36,6 +36,6 @@ steps:
       - apt-get update
       - apt-get -y install openssh-client
       - eval `ssh-agent -s`
-      - echo -n $SSH_PRIVATE_KEY | wc -l
-      - echo -n $SSH_PRIVATE_KEY | ssh-add -
+      - wc -l <(echo "$SSH_PRIVATE_KEY")
+      - ssh-add <(echo "$SSH_PRIVATE_KEY")
       - ssh $SSH_USERNAME@$SSH_TARGET ./deploy-ykonsole.sh $DRONE_BUILD_NUMBER