If you use numbers or other special characters (like +, -, etc.) you need to include the escape character to separate your formula from your text. So you could write that as:
/roll 1d20 + 11 \ for Attack1
Notice the addition of the backslash (\) which tells the dice roller "everything after this is text, don't include it in the roll."
More info on this is documented here:
<a href="http://help.roll20.net/dice-rolling-reference/" rel="nofollow">http://help.roll20.net/dice-rolling-reference/</a>
("Including Additional Information")
