Getting 500 error on fetching orders on Sandbox

[nuruzzamanhimel0]

Hello all, 
I am trying to fetch orders form my sandbox account using Oauth User Token using this endpoint,

https://api.sandbox.ebay.com/sell/fulfillment/v1/order

and getting 

{
  "errors": [
    {
      "errorId": 30500,
      "domain": "API_FULFILLMENT",
      "category": "APPLICATION",
      "message": "System error."
    }
  ]
}  

I actually can't figure it out what am I doing wring in here.
Could anyone please tell me what am I doing wrong in here?