JSTL in IntelliJ gives errors in JSP

Add something like this to your pom.xml under the <dependencies> node (you are using maven, right?):

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>jstl</artifactId>
    <version>2.5</version>
</dependency>

For gradle and other build systems see https://mvnrepository.com/artifact/javax.servlet/servlet-api/2.5

Also, make sure you pick a suitable version for your project. To see all available versions check here.

Make sure that JSTL library jars are added to the module dependencies.