try {
   let res = await fetch('https://pricesea.ru/api/auth/login', {
     method: 'POST',
     credentials: 'include',
     headers: new Headers({'Accept': 'application/json', 'Content-type': 'application/x-www-form-urlencoded'}),
     body: `username=${this.state.username}&password=${this.state.password}`
   });

   if (res.ok) {
     ...
   }
} catch (e) {
   console.log(e);
}

TypeError: Network request failed
at XMLHttpRequest.xhr.onerror (fetch.js:441)
at XMLHttpRequest.dispatchEvent (event-target.js:172)
at XMLHttpRequest.setReadyState (XMLHttpRequest.js:546)
at XMLHttpRequest.__didCompleteResponse (XMLHttpRequest.js:381)
at XMLHttpRequest.js:485
at RCTDeviceEventEmitter.emit (EventEmitter.js:181)
at MessageQueue.__callFunction (MessageQueue.js:260)
at MessageQueue.js:101
at MessageQueue.__guard (MessageQueue.js:228)
at MessageQueue.callFunctionReturnFlushedQueue (MessageQueue.js:100)