Apache Error - sh: svn: command not found

I believe svn is Subversion which is a repository software developers use to store code as they continuously develop new versions of their software. This could mean one of your users is a programmer who is making legitimate software. It could also mean a hacker is trying to do something malicious to your server. I would find out which user is doing it and inspect their files.

It means that someone tried to run the svn command. As it is not present on the server, the error message in question was returned. By the way, this might be a sign of someone trying to attack the website.

As said above by vectro,
"find out which user is doing it and inspect their files."

Log shows that someone tried to run the svn command on your server. If it is not run by your concern then it means someone tried to hack your website. Find out the source of attack and block them