Why is there no sim card option on my iphone
You need to have a SIM card.
Look for / visit a carrier, take a subscription, postpaid or prepaid card.
Check whether the SIM card is suitable for your iPhone.
Then check these Apple articles below:
If you see No Service, Searching, or SOS on your iPhone or iPad
If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
Update your network provider settings manually on your iPhone or iPad
Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)