A deployment or retraction is already under way for the solution

It occurs if anomalies deployment is running for the some package in the backend and some previous deployment has been crashed before due to reason.

To resolve this problem do the next steps:

1.       Run the command prompt
Click "Start", click "Run" type cmd

2.       Go to web server extensions folder
cd C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\BIN

3.       stsadm -o execadmsvcjobs
OR

Locate deployment jobId to break the running process
stsadm-o enumdeployments
Then cancel the deploy
stsadm-o canceldeployment-id "GUID jobId"