403 error page appears when inserting Google AdSense

When inserting Google AdSense in WordPress, the 403 error may occur due to one of the following reasons:

1) Code insertion error
Solution: Please insert the correct code

2) Temporary access issue
Solution: Try accessing again after some time

3) WAF blocking access
Solution: Try setting up after temporarily turning OFF your rental server’s WAF settings

4) If debugging is possible, obtain the sub-code to check error details
Reference: HTTP 403 – Wikipedia