Most devices and communications protocols do not support arrays. Used in IO Tags if IO device supports arrays. Refer to your Device Manufacturer's manual and the specific device driver guide.
Try using a Block if you need an array-like feature for IO tags read from your device.
Text Constant Tags and Text Local Screen Tags can be defined as arrays, where one tag can specify an array of values, each element accessed using tag(number). An array is commonly used by programmers in looping programs or indexed calls. Constant Tag arrays allow users to reference a global tag using the array convention tagname(index) in scripts or programs.