Javascript Errors message – Opt-in form

Hi I want to show 3rd party digital Ads on my website and I installed an Ad Inserter plug-in today. I have added some Javascript code to the plug-in to create an Ad before content (in a post -Ad size 336 x 280) but I am receiving Javascript Errors message. I’m being told that the Opt-in form (on the same page) is causing the error – Error message is:

placeholder.js?ver=1.3.1:47 Uncaught TypeError: Cannot read properties of null (reading ‘addEventListener’)

Cause by script from plugin optin forms: https://www.afrowomanonline.com/wp-content/plugins/optin-forms/js/placeholder.js?ver=1.3.1

Can you please provide any advice on how to fix this error with the opt-in form?

Thanks in advance

The page I need help with: [log in to see the link]