There is a forum rule that new users cannot post more than three times in a single topic without someone else replying to their post. This is to limit spammers who may flood a topic with posts. (There’s also a rule that nobody can post more than three consecutive posts in a topic, but that’s not the case here.)
Users gain trust level on the forum as you read, post, etc. In order to post more than 3 times in a topic without being replied to, you need to be at least Trust Level 1 (Basic User). You also need to be TL1 to send personal Messages, post attachments, and to flag posts. New members start out at TL0 (New User). You can see your trust level at the top of your user profile.
To attain TL1, you must normally post 5 times, read 30 posts and spend 10 minutes reading.
I have merged your reply above to the original thread and manually bumped you up to TL1, so this shouldn’t be an issue for you going forward.