There are several different reasons why you cannot post to a community. You should first make sure that you have joined the community you wish to post in, instead of just adding it to your Friends list. Most communities do not allow posting by non-members, and as such, you will first need to join the community. Once you do so, you will generally be able to post.
If you have already joined the community, it may be that you do not have posting access. If the community is not listed in the "Posting Access" list at the bottom of your full userinfo, but is listed in your "Member Of" list, then you will need to contact the community maintainer and request that you be granted posting access.
If you recently created a community and are the maintainer, you will not automatically receive membership or posting access to your community. You will need to grant yourself access to the community before you can post.
If you recently joined the community and are trying to post from your downloadable client, you may need to log out and log back in so that your client can recognize your posting access to the community.
MODERATED COMMUNITIES
If you receive the error "Maximum queued posts for this community+poster combination reached," then it means that the community you are trying to post in is moderated. In a moderated community, entries are not added directly to the community. Rather, they are put into the moderation queue, which is a list of entries that the community moderators must approve or reject before they can be posted. If you already have an entry in the moderation queue -- that is, if you wrote an earlier entry to the community, and you have not yet received an e-mail saying it was approved or rejected -- then you will not be able to post another entry, and will receive this error. You will need to wait until a moderator has approved or rejected your earlier entry.
The error "Moderation queue full" may also occur with moderated communities. This error message indicates that the queue of posts for the moderator to approve or reject is full; there can be up to 50 entries in the queue at one time, but no more. No one will be able to post any new entries until a community moderator has dealt with some of the existing queued entries.
If a moderator rejects your entry to a moderated community, please do not reply to the e-mail you receive, as that will open a LiveJournal Support request. Support volunteers cannot help you with reasons why your post may have been rejected or help with getting it approved. If you feel that the information provided by the moderator in the e-mail you received was not a sufficient explanation, you will need to contact the moderators directly using any contact information they may have provided on the community's Community Info page.
FURTHER READING
How do I post in a community?
http://www.livejournal.com/support/faqbrowse.bml?faqid=76
How do I join a community?
http://www.livejournal.com/support/faqbrowse.bml?faqid=101
How do I add users and grant posting access to my community?
http://www.livejournal.com/support/faqbrowse.bml?faqid=80
What is a downloadable client? Where can I get one?
http://www.livejournal.com/support/faqbrowse.bml?faqid=158
Your Full User Information Page
http://www.livejournal.com/userinfo.bml?user=exampleusername&mode=full (replacing exampleusername with your username)
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