Constraints are actually evaluated only when a modification on that field is done. when your field is required param set, you cant save the form until it is filled. You should define the company id on the desired form first.

Another option is, override the orm function write() and check the dict contain the value company_id, if not raise the error.

I find a way to work. But it's not perfect.Like this:

Would you please give me some advise.tks

@api.constrains('license_plate')
def _validate_plate(self):    
    res = self.env['fleet.vehicle'].sudo().search([('license_plate', '=', self.license_plate)])    
    if len(res) > 1:        
        raise ValidationError('This license plate already exist')    
    else:                
        if not res.company_id:                        
            raise ValidationError('You must choose company!')

@api.constrains('company_id')
def _check_company(self):    
    if not self.company_id:        
        raise ValidationError('You must choose company!')