When you use the = sign instead of a response.write, you are limited to writing only a single thing. Now, this could be anything from a single integer amount to the result of a mathematical function, or a single string variable to several concatenated together. The error is occuring because ASP doesn't know wether you are trying to response.write a variable or assign one. If you are trying to get a true/false answer, you will need to rewrite a bit more, if you are simply trying to print and assign in the same statement, you will need to break it up into seperate statements.
-Tarwn