I registered on this forum on Feb 5, 2014 and posted quality threads but still I am not able to create signature.
Here is what It says":
Sorry, you are not permitted to have a signature.
Any other rule I need to follow?
No, that’s the only rule. After 90 days, the option should appear. If you tell me what you want in your signature (either post here or send me a PM), I’ll see if it will let me set it up for you.
Unfortunately, the vBulleting software we’re using is developing all sorts of glitches, but as we’ll be moving to Discourse in the near future, there’s no point in trying to fix it now.
how to get a constant signature to follow some rules?
As you may have deduced from the above posts, you need to wait 90 days for that option to become available. You can find the guidelines here: http://www.sitepoint.com/forums/faq.php?faq=youraccount#faq_signatures
If, however, we move to Discourse before then, the whole issue will become irrelevant, as Discourse doesn’t support signatures.