According to JSR223 Sampler documentation:
JMeter processes function and variable references before passing the script field to the interpreter, so the references will only be resolved once. Variable and function references in script files will be passed verbatim to the interpreter, which is likely to cause a Syntax error. In order to use runtime variables, please use the appropriate props methods, e.g.
props.get("START.HMS");
props.put("PROP1","1234");
So amend the last line of your script to look like:
log.info(vars.get('counter'))
Demo:
还请注意，它使用起来容易得多：
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