You need to reach Trust Level 1 in order to flag posts. Assuming that the settings on this forum are the same as the defaults, then you need to read 30 posts to get there. Your profile says you've only read 14 posts so far, so once you read some more of the forum you should get promoted to that trust level and gain the ability to flag.
It is a bit odd, though, that you get a message saying to submit a flag when you don't yet have the ability to flag. That may be a Discourse bug (or misconfiguration)?