Why can't I add a new answer on someone's question??

When questions get a bit old they get automatically locked. So that is probably why.  Look for the lock symbol.

 

If you want an old question unlocked so that you can answer it, just send a moderator a privae message, with a link, and ask them to unlock it for you. 

Don't forget the please :) 

 

Note:  it will not stay unlocked for ever.