I've premium wp themes,but some of them always come with error,such as line 7 error,,etc any suggestion to fix it?
That's a pretty broad question. There is really no way to answer without seeing the theme, the site, or something else more than just saying "premium themes". There are thousands of them. In my experience, errors on installation come from not following ALL of the set up directions.
It's most likely a PHP error you are experiencing. Some variable is not being defined, or there are floating PHP tags. Try entering the page where the error is occuring through the Appearance->Editor link. Then, count down 7 lines and see what is coded on that line. If it's a variable, $example, you may need to give this variable a value in your theme options. You may also have an unexpected PHP opening or closing.
Some designers sell premium Wordpress themes without support. For this reason, you should buy from a trusted designer/developer that is willing to stand by their product. Some people are simply out to make money...