ADB not finding device with Galaxy S8 and Windows 7
I suggest you to connect your phone to your PC, and on your computar to go on Device Manager in control pannel and remove devices related to your smartphone.

Then, disconnect the smartphone from you PC.

Restart your PC and your phone.

Connect again your Phone to your PC.

Another try:
Simply disable USB Debugging, while connected to PC.
Re-enable It and connect to your PC.