It may happen that JMeter scripts that run well locally fail in Blazemeter. When checking the jmeter.log or bzt.log, contained in artifacts.zip (downloadable from the report's logs tab), following error may be seen:
2021-01-07 12:14:27,112 ERROR o.a.j.JMeter: Error in NonGUIDriver
java.lang.IllegalArgumentException: Problem loading XML from:'/tmp/artifacts/modified_Jmeter1.jmx'.
This error means that the JMeter instance install on the Blazemeter test engine does not include the plugin listed in the last line, in above case the ParallelSampler. While Blazemeter automatically includes standard plugins that are typically available through JMeter's Plugin Manager, a few non-standard plugins must be uploaded with your test in order to run.
In order to correct the error, simply upload the plugin JAR file along with your JMeter script, as you would any other test file, per the instructions found in Uploading Files & Shared Folders.
In the example above, the jmeter-parallel-0.*.jar that the local JMeter apparently has installed in the ./lib/ext dir, should be uploaded. Because the local JMeter has access to that plugin but Blazemeter has not, the test runs well locally but fails in Blazemeter.