OllyDbg is interpreting numbers as hex by default. But 0x55000 is larger than 0xFFFF so it cannot be stored in a 16bit register, which ax is, hence it complains.
If you meant the decimal number 55000, I think you can enter it as "55000." with the trailing dot. If that doesn't work enter the number as hex:
mov ax, 0D6D8
The leading 0 is necessary otherwise ...