Hold a random number
You are redefining the random number wrongly each time with

randomNumber += xPrevious;


this is an error. You need another new variable (initialised at 0) to keep the running totals of all the random "rolls". You should be able to fix your program now you know this. Maybe something like

TotalofTherandomNumbers += xPrevious;


to start off with.

You may also want to add in the after the line

 randomNumber = random(1,7);


a new function, of yours, that takes random value randomNumber  do the action in the game, what ever that is,  depending on if its the first or second player.