Iāve seen this as well. The behaviour seems somewhat inconsistent, when I tried to replicate it a couple of days ago it allowed a reply,
Just had the same thing occur attempting to reply to a thread:
Same thing happened both when I quoted that line, & also just hitting the reply button?
Edit: But trying again by highlighting that line, then pressing reply (which is what I did previously) worked the next time! 
Just had it happen again, after I had quoted a line to reply to. Deleted the quote & just replied & it worked, so that may be the issue?
Were all those issues in Help and support and Tagging help & support (i.e. potentially related to the new Q&A plugin) or do they occur in normal threads as well?
Testing quote
Which worked so itās OK here!
Sorry, I couldnāt say whether they were or not? Iāll have to see next time it happens
Iāve seen it a few times (includng yesterday), always in areas with comments enabled. See also this.
Same here on that exact post too.
Edit: I think perhaps weāre supposed to ācommentā on the reply we want to respond to, rather than creating our own new response?
Apparently this is exactly what happens. If you add a general reply to the topic using the blue āReplyā button at the end everything is fine. You can then even quote as many passages out of other posts as you like.
If you reply directly to the post of another user using the reply button under the post you will get the error messages because Discourse expects you to use the comment function instead.
I had the same error message and when I tried the different options this was the result.
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Just had a similar sort of an issue here: Irrelevant locality names - #3 by SimonPoole
Teknoboy added a comment to ask a question.
When I then added a comment to reply to them, it told me that Comments are limited to 600 characters, which isnāt a lot.
I then added a reply Irrelevant locality names - #7 by Fizzie41 but there was no way of actually quoting his comment?
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Comments seem to be in general somewhat a 2nd class citizen in Discourse. In any case it would be nice if there was a way to convert comments to regular replies and the other way around.
Oh yes, and this is going to get some knickers in a knot: there doesnāt seem to be a mechanism to report (or moderate in general) comments.
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ā¦ or even just select text and just click āquoteā, actually, or as @dieterdreist found, reply by email.
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Correct, by doing so a direct reply to the post is startet, attracting the error message. Havnāt tried the email thing but Iām sure @dieterdreist is correct.
Looks like the only thing you can do is to copy text out of a comment and paste it into the reply window. If you want it to look good, you can even add the quote manually ā¦ like in the good old days ā¦ 
), for instance
The plugin being used for Q&A is a great candidate for exploring if its development can be āsponsoredā by OSMF and prioritize solving some of the issues commented on this topic.
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I have it too, see here and follow-up reply and itās comment.
I think it is some unlucky configuration here on community.openstreetmap.org (āunluckyā because the error message is confusing, only appears after having written the answer, and does not say what to do instead).
If it really is not wanted to post answers on answers, then I think it is important to make the user experience better: Tell straight away that it does not work and what do do instead.
Comments are also not a good solution since apparently they do not support every formatting (see ābrokenā quote in the comment to this post) and they allow only up to 600 characters, making it difficult to express more complex stuff. Another suggestion is to allow comments to be as complex as posts.
a workaround is muting the Help section, this way you are not tempted to reply
Actually, I asked a question, and a reply asked something back, so I was due to reply.
An alternative might be to actually read the message before replying to it, and if itās from āHelp and supportā, donāt reply to answers by email
That requires background information not easily available. You would need to know the specialities how this category. If this category behaves as it behaves, the functionality to reply to a post should just not be offered in the first place, if it does not work! I consider it as a bug if something is offered which at the end is declined.
Yes, my email also just got /dev/nulled. Please add a bounce message in that case!
ā¦ other than in the subject line of the email? 
OK, while āHelp and supportā is in the subject line of every email from that section, strictly speaking people could turn post voting off there (or turn in it on in some other category), in practice no-one does, so you can tell which emails are OK to respond to and which are not.
That sounds like a feature request that is best directed at the authors of Discourse. It is not maintained by OpenStreetMap - we have enough to do mapping everything on the planet.