str is a build in function to convert a value to a string. So can't use it as a variable name. But you can use it as a string.
I guess you want a list of skills with values. And then to get a random set from the combined list?
var sk1 = ["str",1]
var sk2 = ["agi",2]
var array = [sk1,sk2] #returns [[str, 1], [agi, 2]]
var rngsk = array.pop_back() #pop_back is faster than pop_front
print(rngsk) # returns [agi, 2]
print(rngsk) #returns agi
print(rngsk) #returns 2