Hello Dr Sun,

While I was trying to second homework, I am frequetly getting the same error.

I am feeling like some library  is missing as shown in error message

Thanking you

The first part of the error message ("URI ... is already defined") seems to mean that you have two copies of JSTL. My guess is that you have the jar files both in the Tomcat lib folder and your project lib folder. This should be OK though.

The 2nd part of the error message ("/paer.jsp ...") says you specified an expression to an attribute "value" which cannot take expressions. This is a problem of your code, not because of some missing library.