Looking on the internet deeply has found these results:

no problem is the most popular phrase on the web.

not a problem 12,620,000 results on the web	no problem 1204,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Jan 12, 2015 ... [It] is not [a problem][to do that]. (we added the pronoun "it". A pronoun, speaking simply, is a word that is used as a stand Apr 20, 2007 ... I'd normally say "it's no problem", but now that I've heard some chap saying "it's not a problem" I'm no longer sure whether "it's no problem" is�... in for a noun. May 27, 2013 ... ... someone born before 1980, just keep telling him "No problem" when they ask you to do something that is most certainly NOT a problem. Jul 3, 2012 ... Many people seem to have adopted the expression "Not a problem" in place of " You're welcome." For example, if I thank someone in email,�...	You had no problem cursing me. My people can handle this, no problem. law comes to visit... no problem. Cody has no problem accepting what's offered. Mother in I have no problem with alternative lifestyles. 500, alright, no problem.

not a problem

12,620,000 results on the web

no problem

1204,000,000 results on the web

More popular!

Jan 12, 2015 ... [It] is not [a problem][to do that]. (we added the pronoun "it". A pronoun, speaking simply, is a word that is used as a stand
Apr 20, 2007 ... I'd normally say "it's no problem", but now that I've heard some chap saying "it's not a problem" I'm no longer sure whether "it's no problem" is�...
in for a noun.
May 27, 2013 ... ... someone born before 1980, just keep telling him "No problem" when they ask you to do something that is most certainly NOT a problem.
Jul 3, 2012 ... Many people seem to have adopted the expression "Not a problem" in place of " You're welcome." For example, if I thank someone in email,�...