ArgumentError: Mapper mapped class Roles_users->User Roles could not assemble any primary key columns for mapped table 'User Roles' - roles

Question

I'm trying to write a web app with user roles and I keep getting this error no matter the changes I make.
Could you please help me find what's wrong?
Thank you in advance!!
This is my models.py file:
from sqlalchemy import Column, Integer, String, Float, Boolean, ForeignKey
from flask_login import UserMixin
from sqlalchemy.orm import relationship, backref
import db
class Rol(db.Base):
__tablename__ = 'Roles'
id_rol = Column(Integer, primary_key=True)
nombre_rol = Column(String, nullable=False)
user_rol = relationship("User rol", back_populates="rol", uselist =False)
def __init__(self, nombre_rol):
self.nombre_rol = nombre_rol
class User(UserMixin, db.Base):
__tablename__ = 'Users'
user_id = Column(Integer, primary_key=True)
username = Column(String, unique=True, nullable=False)
password = Column(String, unique=True, nullable=False)
is_active = Column(Boolean, default=False)
user_rol = relationship("User rol", back_populates="user", uselist =False)
def __init__(self,username,password,is_active):
self.username = username
self.password = password
self.is_active = is_active
def get_id(self):
return self.id
def is_active(self):
return self.is_active
def activate_user(self):
self.is_active = True
def get_username(self):
return self.username
def get_rol(self):
return self.rol
class Roles_users(db.Base):
__tablename__ = 'User Roles'
user_id = Column(Integer, ForeignKey("Users.user_id"))
id_rol = Column(Integer, ForeignKey("Roles.id_rol"))
user = relationship("User", back_populates="user roles")
rol = relationship("Rol", back_populates="user roles")