Incompatible types in assignment of 'uint8_t {aka unsigned char}' to 'uint8_t [1] {aka unsigned char [1]}'
atoi is for converting a textual representation of a number ("123" for example) into an actual integer.

The c_str() function gives you a pointer to the internal buffer of the String (assuming you actually have a String) which is no different to a uint8_t[] or uint8_t * (other than the signedness). 

Without knowing exactly what the destination function for this buffer requires it's very hard to help you, but you may want something like:

LoRaWan.send((uint8_t *)myString.c_str(), myString.length());


If you need to copy the data into a fixed length buffer then you need to first create a buffer of that actual size - then copy the string data into it:

// Create a buffer of 32 bytes
uint8_t myBuffer[32];
// Copy at most 32 bytes, but no more than there actually is, into the buffer
memcpy(myBuffer, myString.c_str(), min(myString.length(), 32));