Place a goto Main at the end, and change ["255"] to [255].
Code:
Main:
    SerIn Si,4,[255],pinstate
    IF pinstate=1 Then 
      GoSub Relayon
    Else 
      GoSub Relayoff
    EndIF
    GOTO Main ' <-- forgot this

Relayon:
    High RELAY
    Return

Relayoff:
    Low RELAY 
    Return