To start with, use the examples ttn-abp.ino and ttn-otaa.ino that are included with the LMIC library that you are using.
To prevent running into version incompatibilities: use the examples included with your library and do not use examples from a different version of the same library or from a different library/repository.
As @cslorabox and @descartes already mentioned you may quickly run out of memory when using MCCI LMIC with ATmega328 MCU boards like a Pro Mini (when doing anything more than bare basics). If so then the older Classic LMIC may be a good alternative.
(That said, memory size is probably unrelated to your current issue.)
Also see: Arduino LMIC library naming: Classic LMIC and MCCI LMIC [HowTo]