Error: A deployment or retraction is Already under way for the solution 'solution.wsp', and only one deployment or retraction at a time is supported.

Sharepoint sometimes surprises us with some unexpected errors

Now try to solve one that says: A deployment or retraction is Already under way for the solution 'solution.wsp', and only one deployment or retraction at a time is supported.

The message is correct and explains the limitation that has sharepoint. This happens when you want to deploy a package, while another is being deployed at the time.

Before deleting the job run, analyzing whether to delete it, because the the deployment may be valid

Please try to run the following commands:

1. Look for the deploy jobId to break the running by the following command: stsadm -o enumdeployments 
2. Cancel the deploy typing the following command. stsadm -o canceldeployment -id "jobId" 
3. Check that has been canceled deploy by typing the following command: stsadm -o enumdeployments 

And ready to deploy again and it should (remember is sharepoint) run