randomSeed()takes one paramter - the seed to the pseudo random sequence
on a PC I tend to use time the arduino documentation recommends
randomSeed(analogRead(0));
Are you attempting to generate a sequence of random characters?
if so use the random() function to randomly access an array of characters 'a' to 'z'?
or generate a random number between 0 and 25 and add it to 'a'