JMeter script doesn't load

[Alexey-Zheltov]

JMeter throws exception when trying to load sample script:

Problem loading XML from:'C:\load-tests-main\jmeter-script\AuthCodeFlowTest.jmx'.
Cause:
CannotResolveClassException: kg.apc.jmeter.threads.SteppingThreadGroup

Detail:com.thoughtworks.xstream.converters.ConversionException:
---- Debugging information ----
cause-exception : com.thoughtworks.xstream.converters.ConversionException
cause-message :
first-jmeter-class : org.apache.jmeter.save.converters.HashTreeConverter.unmarshal(HashTreeConverter.java:66)
class : org.apache.jmeter.save.ScriptWrapper
required-type : org.apache.jmeter.save.ScriptWrapper
converter-type : org.apache.jmeter.save.ScriptWrapperConverter
path : /jmeterTestPlan/hashTree/hashTree/kg.apc.jmeter.threads.SteppingThreadGroup
line number : 84
version : 5.6.2