During a POST request (if I understand correctly, this is supposed to be a registration), an error of 500 is returned. I suggest that you consider making registration a separate endpoint just like login.
POST https://flapp.sytes.net/api/v1/users/
Content-Type: application/json
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ0b2tlbl90eXBlIjoiYWNjZXNzIiwiZXhwIjoxNzAyOTgyNzczLCJqdGkiOiJiZmJlYjY3YjE4MmY0YzFhOTg2YTcyZWYxZWM0ZDMxNiIsInVzZXJfaWQiOjJ9.BJe_SFBaRYx5qED1_A739wcSfBuaD1o-OA8U4zIEPH0
{
"email": "cat@example.com",
"username": "cat",
"first_name": "cat",
"last_name": "catlast",
"password": "CATcat11@@",
"gender": "male"
}

During a POST request (if I understand correctly, this is supposed to be a registration), an error of 500 is returned. I suggest that you consider making registration a separate endpoint just like login.