asp.net core 3 identity generatePasswordResetTokenAsync causes internal error

I have setup the following code:

[HttpPost("requestPasswordReset")]
[AllowAnonymous]
public async Task<IActionResult> requestPasswordReset(UserNameDto userNameDto)
{
    var user = await _userManager.FindByNameAsync(userNameDto.Username);

     if (user != null)

    {   
       var code = await _userManager.GeneratePasswordResetTokenAsync(user);
      return Ok(true);
    } 
  return Ok(false);

"var code = await _userManager.GeneratePasswordResetTokenAsync(user)" code returns an internal error with no more information. Could someone help?