I would suggest going over the section on how to hold the guitar and make sure you have a suitable posture. Check that the head of the guitar is about the height of your shoulder and the neck of the guitar is angled a little forward. You may need to adjust how high the guitar is. If it is too low. this can prevent the arm and wrist attaining the most comfortable position. The use of a strap is recommended in both the standing AND sitting position. Make sure your fingers are curled ( imagine holding a tennis ball). Use the tips of the fingers and keep your nails very short on your fretting hand.While fretting notes on the guitar, your fretting wrist should only bend moderately as you curve your fingers to reach for strings.

If your wrist is too painful then you should rest it for a while to avoid injury.

I hope this is of some help.

Kind regards

Joe