bool allChecked;

if (l1tag == 1) allChecked = ...;
else if (l2tag == 2) allChecked = ...;
// continued...

if (allChecked)
{
    rannum = randNum.Next(101);
    textBox1 = rannum.ToString();
    i++;
}
else
{
    textBox1.Text = "Not Eligible for Roll";
    i++;
}

?

Edit: Frank has a way better solution, it depends how seriously you want to take the architecture.