Flashing your Samsung Galaxy S8 to solve an "Invalid SIM" problem can be a bit technical, but here’s a step-by-step guide to help you through the process. Make sure to back up your data beforehand, as flashing can erase everything on your device.

Prerequisites

Backup Your Data: Ensure all important data is backed up.
Charge Your Device: At least 50% battery life is recommended.
Download Necessary Tools:
- Odin: A tool used for flashing Samsung devices.
- Samsung USB Drivers: Install these on your PC.
- Stock Firmware: Download the correct firmware for your Galaxy S8 model from a reputable site like SamMobile or Updato.

Steps to Flash Samsung Galaxy S8

Install Samsung USB Drivers:
- Connect your S8 to your PC and install the drivers if prompted.
Download and Extract Odin:
- Download the latest version of Odin and extract the files to a folder on your computer.
Boot Your Device into Download Mode:
- Power off your Galaxy S8.
- Press and hold the Volume Down + Bixby + Power buttons simultaneously.
- When a warning screen appears, press Volume Up to continue to Download Mode.
Launch Odin:
- Open the Odin tool on your computer.
Connect Your Device:
- Connect your Galaxy S8 to your PC using a USB cable. Odin should detect your device, and you’ll see a blue or green box in the ID
  section.
Load the Firmware:
- In Odin, click the AP button (or PDA on older versions).
- Select the firmware file you downloaded (it should have a .tar.md5 extension).
Start Flashing:
- Make sure that "Auto Reboot" and "F. Reset Time" are checked in Odin.
- Click the Start button in Odin to begin the flashing process. Wait until it finishes; you’ll see a “PASS” message in green.
Reboot Your Device:
- After flashing is complete, your device will reboot automatically. If it doesn’t, you can manually reboot it by holding the Power button.

After Flashing

Check SIM Functionality: Insert your SIM card to see if the "Invalid SIM" issue is resolved.
Restore Your Data: If the flash was successful, restore your data from the backup.

Important Notes

Warranty: Flashing your device can void your warranty.
Risk of Bricking: If not done correctly, there’s a risk of bricking your device.
Firmware Compatibility: Ensure the firmware is specifically for your model (check the model number in Settings > About phone).

If you're unsure or uncomfortable with these steps, it might be best to seek help from a professional technician.