It seems like SCP plugin is using SFTP to send publish artifact over SSH :

[SCP] Connecting to *****
ERROR: Failed to upload files
com.jcraft.jsch.JSchException: failed to send channel request
at com.jcraft.jsch.Request.write(Request.java:65)
at com.jcraft.jsch.RequestSftp.request(RequestSftp.java:47)
at com.jcraft.jsch.ChannelSftp.start(ChannelSftp.java:190)
at com.jcraft.jsch.Channel.connect(Channel.java:200)
at com.jcraft.jsch.Channel.connect(Channel.java:144)
at be.certipost.hudson.plugin.SCPSite.createChannel(SCPSite.java:163)
at be.certipost.hudson.plugin.SCPRepositoryPublisher.perform(SCPRepositoryPublisher.java:161)
at hudson.tasks.BuildStepMonitor$3.perform(BuildStepMonitor.java:45)
at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:770)
at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:734)
at hudson.model.Build$BuildExecution.post2(Build.java:183)
at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:683)
at hudson.model.Run.execute(Run.java:1765)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:43)
at hudson.model.ResourceController.execute(ResourceController.java:89)
at hudson.model.Executor.run(Executor.java:240)
Build step 'Publish artifacts to SCP Repository' changed build result to UNSTABLE

I would use this plugin because my server doesn't handle SFTP but does handle SCP.
When configuring SCP host, everything seems ok, aka the plugin can connect to the server. But when sending a file from a job, the error above appears.
Job configuration attached