That message @Bittu usually means the sim card is not communicating with it's network.
The sim card maybe glitching.
Can you eject it and give the chip on it a wipe with a soft cloth and then make sure it's properly seated when re inserted.
As a process of elimination can you try your sim card in another phone and another sim card in your phone ?
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.
Samsung have started rolling out a bugfix update with the Android Security patch for January, for both the S20 FE 4G and the S20 FE 5G that will hopefully fix your issue.