The first step is to cast to resolve the ambiguous operator. Next, rearrange your query to get rid of the two extra subqueries:
Int64 memberId = reader.GetInt64(0);
var user = Context.Guild.Users
.Where(x => x.Id == (UInt64)memberId)
.First();
string name =
user.Nickname != null
? user.Nickname
: user.Username;
Int64 votes = reader.GetInt64(2);
GOTWVote.Add($@"{name} has received {votes} vote(s)");
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…