services.AddSingleton<IMapper>(new Mapper(new MapperConfiguration(cfg =>
{
 cfg.CreateMap<User, MentionUserDto>();
})));

public class MappingProfile : Profile
{
 public MappingProfile()
 {
  CreateMap<User, MentionUserDto>();
 }
}

services.AddAutoMapper();

.Take(10)
.ProjectTo<MentionUserDto>()
.ToListAsync(); 

Mapper not initialized. Call Initialize with appropriate configuration.

.ProjectTo<MentionUserDto>(_mapper.ConfigurationProvider)

services.AddSingleton(config);
return services.AddScoped<IMapper>(sp => new Mapper(sp.GetRequiredService<IConfigurationProvider>(), sp.GetService));

AutoMapper.IConfigurationProvider config = new MapperConfiguration(cfg =>
{
 cfg.AddProfile<MappingProfile>();
});
services.AddSingleton(config);
services.AddScoped<IMapper, Mapper>();