Faulty monitor code.

Hello everyone.

Code link

I'm currently working on a program for a playershop i'm currently making, to display item type, price and amount.
However i get this error: attempt to index ? (a number value) at line 21

In my head it makes sense, but the computer will not accept it.
I'm just stupid… but i would be delighted if somebody could point out the case and or come up with a solution to the problem.

// Bowling25

Look a little closer at line 21.

  center(offset,blockno-->.<--shopam[amount],3,charoffset)

Lol thank you very much:)/> how could i be so blind…
I have just been swearing at this code for a couple of hours -.-.