Hi,
What is the model number of the tablet?
A SIM card is only necessary if you wish to connect to a mobile phone service with the tablet for either a phone or data connection.
Normally lesser priced tablets don’t have a SIM card feature and use WiFi to connect for data connections.
They do have a SD card slot to increase the storage in the tablet though. Maybe this is what you meant. The SD card isn’t necessary to make it work however