Your code looks remarkably similar to Nayeli’s (Code Queen) code from her tutorial here.
https://codequeen.wixsite.com/membership-dashboard
If you are taking her code example and modifying it to suit your own needs, then I would suggest that you contact her directly if you have any issues with it. Then if she can’t help you, then please come back to this forum and ask for more help and someone will try to help you.
As for your error link, well that is now fixed according to Wix, so in theory you shouldn’t have the error still persisting with your website. However, if you do then please follow Mike’s prefect response and check that your firstName field does actually contain a value that the code can use.