There are two places where you can find the whiteboard on Google Meet.  To illustrate, let us look at the Google Meet toolbar.

There are several icons to be found in the toolbar located at the bottom of the screen but to record a Google Meeting, you can go through ACTIVITIES or MORE OPTIONS.

Activities

The ACTIVITIES icon is the second icon from the right on your screen that looks like a triangle stacked on top of a square and circle.  When you click on that the ACTIVITIES panel will appear displaying two items: RECORDING and WHITEBOARDING. Click on the WHITEBOARDING button to trigger the whiteboard.

More options

The MORE OPTIONS button is among the set of icons found in the middle of the bottom of the screen and to the left of the END CALL button.  It appears as three dots stacked on top of each other.  In the illustration above, click on the second link from the top marked WHITEBOARD to trigger the whiteboard.  Sometimes the CAST THIS MEETING button does not appear, so the WHITEBOARD button will appear on top.