Yes, you have credit, but you have a 0 RAC. It requires an active cruncher to post in there, so your RAC must be at a certain limit before you can post in there, and must maintain that to keep posting in there. Once it drops below that amount, it takes it away, again.

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...in order to post at the Science forum. How much is "a certain amount"? Or should it say "an uncertain amount"?
I thought it would be one unit, but I already completed a unit and still won't let me post...

(And I used to have, I think it was 3000 or 4000 units with my old Seti Classic, just that I couldn't remember what my email was back then. And, to be honest, I only joined again for the sake of chatting at the Science forum; I don't really like Seti BOINC... With Classic there were all kinds of nifty add-on's one could use to check out where exactly on the sky the unit was taken from, the path of the radiotelescope, etceteras. All gone.)

Boinc is not Seti-Classic and Classic credits are not compatible with Boinc's so they would be shown separately in your account. If you look on the left of your post you will see that you have not yet received any credits from Boinc-Seti.
Boinc has a two step Reporting Process. The finished result are uploaded immediately but reported later when Boinc contacts the project again. Issuing Credit is also not automatic since only validated results are granted credits.
I believe you only need a RAC (Recent Average Credit) of one to be allowed to post. This is to ensure that only active crunchers can participate in certain forums and keep the trouble makers out.
I hope this explains things a bit. In the meantime perhaps you may want to learn more about how Boinc works by reading about it with the link in my signature.

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Boinc V7.2.42
Win7 i5 3.33G 4GB, GTX470

...in order to post at the Science forum. How much is "a certain amount"? Or should it say "an uncertain amount"?
I thought it would be one unit, but I already completed a unit and still won't let me post...

(And I used to have, I think it was 3000 or 4000 units with my old Seti Classic, just that I couldn't remember what my email was back then. And, to be honest, I only joined again for the sake of chatting at the Science forum; I don't really like Seti BOINC... With Classic there were all kinds of nifty add-on's one could use to check out where exactly on the sky the unit was taken from, the path of the radiotelescope, etceteras. All gone.)