I updated spamassassin using the sa-update && service spamassassin restart command but got the service: command not founderror message. service is in /sbin but I could not execute it.
Please, check if there is Ubuntu installed at the server. There is no "service" utility at the early Ubuntu versions. That is why you get such response from the server:
service: command not found
In this case the spamassassign can be restarted with the following command: