Error Fetching Inventory Item/items in Sandbox

When I try fetching my inventory Item wether whole inventory or just single 1 inventory item via sku its returns me this error.

[
  {
    errorId: 25001,
    domain: 'API_INVENTORY',
    subdomain: 'Selling',
    category: 'SYSTEM',
    message: 'A system error has occurred.'
  },
  Headers: {
    'content-length': '138',
    cneonction: 'close',
    date: 'Mon, 08 Mar 2021 10:18:54 GMT',
    rlogid: 't6pitnmsgwj%28ciuoopu0asu%60pkal%3C%3Dosutowoiudvn.a2u%3A2%2Bsfvu7371-1781158c73b-0xea',
    'set-cookie': [ 'ebay=%5Esbf%3D%23%5E;Domain=.ebay.com;Path=/' ],
    'x-ebay-c-request-id': 'ri=Bl8btf8j%2B38u,rci=c9639c820899a37d',
    'x-ebay-c-version': '1.0.0',
    'x-ebay-esb-upstream': 'slrinvapi.vip.sandbox.ebay.com',
    'content-type': 'application/json',
    connection: 'keep-alive'
  },
  StatusCode: 500
]

PLUS if we create inventory item, it returns an empty object instead of newly created inventory item.