append(message.Buf, 0xff)
first argument to append must be slice; have *bytes.Buffer
append(0xff,message.Buf)
first argument to append must be slice; have untyped number
我应该如何使0xff作为要附加的切片呢？
You have a buffer which is of type bytes.Buffer (or more specifically a pointer to that type). It has a Buffer.WriteByte() method, just use that:
message.Buf.WriteByte(0xff)
The builtin append() function which you tried to call is to append values to slices. bytes.Buffer is not a slice, you can't use that with append() (it is implemented using an internal slice, but that is an implementation detail which you should not build on / utilize).