Jmeter is giving response as 400, when posting request with data

Use Menu option :

• Tools > Function Helper Dialog

to fix your syntax as it is incorrect it should look like:

 ${__eval(${__FileToString(json file path,,)})}

Add a view result tree and check the request tab yo see if you are sending correctly data.

If yes, then check you are sending correct Content-Type header by adding a Header Manager and adding header:

 Content-Type : application/javascript 

if it’s JSON or another value

1. Your syntax is not very correct, __FileToString() and __eval() functions combination should look like:

   ${__eval(${__FileToString(json file path,,)})}
   

   See Here’s What to Do to Combine Multiple JMeter Variables article for more details

2. You might also need to add HTTP Header Manager and configure it to send Content-Type header with the value of application/json