What have you tried? Bit hard to help without know what has been tried and failed.

But in general this should be how:

To boot your S10 into recovery mode, you'll first need to turn off your device. After you've powered it down completely, press and hold on both the volume up and Bixby buttons (the button just beneath the volume rocker), then press and hold the power button.

If that isn't working, I'd suggest you need to try the combination a few times to get it working. (otherwise what is the exact model you have).

General concern here: if you can't get into download mode, are you sure you should be using it?

i did the  power down plus bixby and power button, even the up vol and bixby and power it wont load boot  mode, unless i need to set some seting in developer opton?  it has knox 3.7 built in it, my phone is mdel number: SM-G977B

Samsung seem to have changed the way to get into recovery & download mode with their latest firmware.

First, power the device down.

Then, plug into USB (note this needs to be plugged into your PC, not just a charger).

Once the charging indicator comes up:

For recovery - hold down vol up + bixby + power. Release power when you see the Samsung logo, the device should boot into recovery mode.

For download - hold down vol down + bixby + power. Release power when you see the Samsung logo, the device should boot into download mode. You can also access download through recovery ('Reboot to Bootloader').