It makes sense that you can’t find the fuel tank door release button, because the 2017 Toyota Avalon actually doesn’t have one!
All you need to do to open the door is gently press the outer edge. However, there is a key factor: your Toyota must be off and unlocked for this to work! If your Avalon is locked, the gas door will be, too.
Once you get the door open, simply twist the gas cap to the left to remove it. Pump your gas, and then replace the gas cap and turn it to the right. You should hear or feel a click once it’s sealed. Then, gently press the door to close it again!
If you experience issues with your gas tank door, head to your local Toyota dealer or a trusted mechanic for assistance.