Following query what you've written will cause error you stated definately because you are getting more then one linenamet from linet, That is 3 for you example.
update line set LineName = (select linenamet from linet);
For that you can modify your query like.
update line set LineName = (select linenamet from linet --include some condition which gives you exact single result from corrosponding table)
If there's a single column in
linenamet
table then I think it is not possible to fetch.