Cricket customer service is having trouble putting the phone in the system.
Are you in an AWS area? Does cricket give a reason they can't add the ESN to the account? What did you flash it with? DFS? CDMAIPHONETOOL? Other solution? Without knowing how it was flashed people can't offer much help and only speculations.
I could speculate and say it's a network problem, and the fact that cricket claims they no longer allow flash iphones to be added to accounts could contribute. Other speculations would be a small error like corrupt PRL, etc. But those are just speculations.