The assembeler tell me "Expression expected"
The only expressions allowed on the right side of '=' are those computable by the ALU, as shown in Figure 4.5. To set RAM to an arbitrary value you need to use multiple statements.
Note that the '@' command only handles numbers in the range 0–32767 (15-bit unsigned integers).
What you need to do is set RAM to the negative number that has the same bit pattern as 65535. There is a legal M= command to do that.