TempData serializer 'Microsoft.AspNetCore.Mvc.ViewFeatures.Infrastructure.DefaultTempDataSerializer' cannot serialize property 'ChallengeHub.Web.Pages.Posts.CreateModel.AlertMessage' of type 'ChallengeHub.Web.ViewModel.AlertMessage'.

Solution

TempData uses Sessions which uses strings, so you need to serialize your data using

TempData["key"] = JsonConvert.SerializeObject(obj);

and then deserialize it later using 
JsonConvert.DeserializeObject<DataType>

https://stackoverflow.com/a/56529252/10548884