If the SIM card is not correctly installed in the tray and inserted into your phone, the SIM card may not be recognized. If the SIM card is not recognized, remove the tray and check if the card is properly installed. It is particularly important to ensure that the corners of the SIM card are well aligned.
When you insert the SIM card tray, notice the mounting direction of the SIM card as it may be different for each model. For example, in case of the S21 series, they are designed with the metal socket facing up, and the S20 series are designed with the metal socket facing down. Refer the image examples below.