Most likely you have a syntax error in your Beanshell script, try using the below reference code which is reading the file and prints its contents into jmeter.log
String test = org.apache.commons.io.FileUtils.readFileToString(new File("C:/Test/test.txt"));
log.info("test.txt file content is: " + test);
Demo:

See How to Use BeanShell: JMeter's Favorite Built-in Component article to learn more about Beanshell scripting in JMeter.
NB: it may be faster and easier to use __FileToString function like:
${__FileToString(c:\test\test.txt,,)}
