Do you have a WiFi Only or WiFi+Cellular model iPad? WiFi Only models do not have a SIM slot or any Cellular hardware:
Remove or switch the SIM card in your iPad - Apple Support
The iPad User Guide is a rich source of frequently overlooked information about your iPad and iPadOS - and includes detailed information for each iPad model, including the specific location of the SIM slot for Cellular models.
The User Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892