Try:
public async Task OnGetAsync(string sortOrder, string searchFCode)
{
string userName = User.Identity.Name;
var userQuery = from u in _context.Users.AsNoTracking()
where u.UserName == userName
select u.FCode;
string fCode = await userQuery.SingleAsync();
var publisherQuery = from p in _context.Publisher.AsNoTracking()
where p.FCode == fCode
orderby p.PubName
select p;
Publisher = await publisherQuery.ToListAsync();
}