I cannot submit new thread on Showcase forums. Is there any restrictions for this area?
You can submit a new thread, and did so several times. The restriction is the same for any area, namely that new users with no post history who use external links in their posts typically have their posts go into the moderation queue (as an anti-spam measure, since spambots always post external links). The limitation is lifted after your posts are approved, or if you’re a spambot then you’re banned. 
–Eric