This is what I see for a protected question:
I had 101 reputation (association bonus), so it surprised me I couldn't answer the question.
Then I found
These state the the association bonus does not count for answering protected questions.
I cannot find any authoritative source (FAQ, blog, etc.) that states this.
This behavior is highly surprising otherwise, since the association bonus counts towards reputation normally in nearly every other circumstance.
The message should be changed to read:
Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site, excluding bonuses.
EDIT: "on this site" does not clarify this. It says I must have "10 reputation on this site". I do. In fact I have 101 reputation on this site. ("Hey Paul, how much reputation do you have on that site?" "101." "How much do you have on that other site?" "13,000.") Surely you wouldn't suggest I have the 100 points extra on another site? (If so, it hasn't shown up yet.)
If it said "10 reputation earned on this site", that would be arguably correct, though still much less clear than my suggested wording.