I think it's a general function or configuration of the forum software rather than some process that's specific to AIRForums, but it seems like you can't send some PMs (maybe ones with links?) until you've had a post or two on the public forum. I see that you signed up in July but didn't post anything until yesterday, perhaps after making a few posts it'll allow you to send a PM now?
Try a test message that has only text in it, no outside links. (you're welcome to send it to me if you like.) Was there any sort of link in the message you tried to send? Moderators have posted in the past that the software's spam trapping seems to dislike new members sending URLs in PMs.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill