How to access Response Status Code 400 as Success? My sampler failed, because the server returns the 400 status code.

You can follow this tutorial to access Response Status Code 400 as Success by adding/modifying the Response Assertion.

If you are intentionally testing a REST API that returns an expected 400 response code, you need to add or modify the Response Assertion of the sampler to have "Ignore Status" check box checked:

[x] Ignore Status

For example, if you want to assert that the response code is 400, you can create Response Assertion with:

Field to Test: Response Code
[x] Ignore Status
Pattern Matching Rules: Equals
Patterns to Test: 400

Or if you want to assert that the response body contains the word "Error" with any response code, you can create Response Assertion with:

Field to Test: Text Response
[x] Ignore Status
Pattern Matching Rules: Matches
Patterns to Test:  .*Error.*

⇒ Recording with JMeter HTTP Proxy

⇐ JSON Extractor - Capture JSON Data

⇑ Working With JSON Body Messages

⇑⇑ Apache JMeter Tutorials