Are you positive you're looking at the SIM slot and not the Micro-SD slot? If you're positive there is a SIM installed - start by rebooting the device. If that doesn't work - contact your carrier, they should be more than happy to set you up with a replacement SIM card to troubleshoot the issue (or try the one you have in another device). If a new SIM card doesn't fix it or the SIM works in another phone, then you need to file a warranty claim with Samsung (but be sure to be clear about the model, the S8 and Note 8 are different devices).
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