What Error 14 means when trying to start an RM126x P2P session?

As per the Application Note - P2P User Guide - RM126x Series, an Error 14 stand for "Command cannot be processed in current state" :

As soon as AT+P2PS is issued (to start a P2P session), the system automatically check all parameters entered to see if your intended P2P network comply with regional regulatory enforcements. 

In general, ATI commands will provide useful information to correct and optimize your P2P configuration :

If an Error 14 comes up it likely means that your actual Window Length is too short and needs to be extended. A good way to know if your Window Length is adequate before starting a P2P session is to check the p2p_minimum_window_length by issuing an ATI 4005 and adapt your Window Length consequently.