.net core Entity Framework The nested query is not supported. Operation1='Case' Operation2='Collect'

Question

I have a following query for some nested implementation

return _context.baseTable1.Where(h => h.id == Id ).Select(lev => new
            {
                Contact = (lev.baseTable2 != null
                                    && lev.baseTable2.baseTable3 != null
                                    && lev.baseTable2.baseTable3.baseTable5.Any(h => h.contact != null)
                                    ? lev.baseTable2.baseTable3.baseTable5.Where(h => h.contact != null).GroupBy(h => h.contact).Select(c=>c.FirstOrDefault().contact) : null),
                Key1 = lev.office != null ? lev.office.id, : null,
                Key2 = lev.baseTable2 != null && lev.baseTable2.baseTable3 != null && lev.baseTable2.baseTable3.baseTable4 != null ?
                                lev.baseTable2.baseTable3.baseTable4.id : null

            }).ToList().SelectMany(x =>
            {
                if (!x.Contact.Any())
                {
                    return new List<FinalModel> { new FinalModel { Key1 = x.Key1, Key2 = x.Key2 } };
                }
                else
                    return x.Contact.Select(contact => new FinalModel()
                    {
                        ContactKey = contact.id,
                        Key1 = x.Key1,
                        Key2 = x.Key2
                    });
            });

It was created via help from GroupBy then Select with conditional record addition

But now getting above error