JWT Fails with Individual Key on App Store Server API

We want to use the App Store Server API for in-app purchase validation. Due to circumstances, we cannot use the team-based private key (team key) and must use an individual authentication key.

However, the JWT authentication keeps failing when using the individual key. Could there be an issue with our code?

  $jwtHeaderArray =                                                       

 [                                                                       

     'alg' => 'ES256',                                                   

     'kid' => $this->KID_ID,                                             

     'typ' => 'JWT'                                                      

 ];                                                                      

                                                                         

 $jwtBodyArray =                                                         

 [                                                                       

     'sub' => 'user',                                                    

     'aud' => $this->ACCESS_TOKEN_AUD,                                   

     'iat' => time(),                                                    

     'exp' => time() + 3600,                                             

     "scope" => [                                                        

         "GET /inApps/v1/transactions"                                   

     ]