Erro na inclusão das excepition

Olá, Davson!
Tudo bem?

Faltou o "\" no início das classes que importou.

Importa elas em cima:
use Tymon\JWTAuth\Exceptions\JWTException;

Exemplo:
use JWTAuth;
use Tymon\JWTAuth\Exceptions\JWTException;

class AuthenticateController extends Controller
{
    public function authenticate(Request $request)
    {
        // grab credentials from the request
        $credentials = $request->only('email', 'password');

        try {
            // attempt to verify the credentials and create a token for the user
            if (! $token = JWTAuth::attempt($credentials)) {
                return response()->json(['error' => 'invalid_credentials'], 401);
            }
        } catch (JWTException $e) {
            // something went wrong whilst attempting to encode the token
            return response()->json(['error' => 'could_not_create_token'], 500);
        }

        // all good so return the token
        return response()->json(compact('token'));
    }
}

(https://github.com/tymondesigns/jwt-auth/wiki/Creating-Tokens#creating-a-token-based-on-the-users-credentials)