For a start you've only defined 63 byte Arrays... and then you try to read 64 bytes into it! Try...

COUNTERx var byte[64]
COUNTERy var byte[64]

Whilst arrays are numbered starting from ZERO, the definition has to contain the actual size... because...

COUNTERx var byte[0]

... is an illegal definition as you've defined ZERO elements for the array... so you need to define at least one element...

COUNTERx var byte[1]

...in order to access COUNTERx(0).