AD621 spice model seems to be not available in models library.
Could you tell me which amplifier spice model could be used instead of AD621 amplifier please ?
You can use the AD620 SPICE model, however since it does not have fixed gains, you will need external resistors to set the gain. The specs are very close to the AD621, although the AD620 has tighter offset voltage specs (depending on the grade).
What requirement do you have for your amplifier? If you need fixed gains of 10 and 100, I recommend you take a look at the AD8228. It is one of our newer parts and has better specifications. It also has a fully usable SPICE model which would be beneficial for you.