Custom CSS produces a broken link image

I'm trying to put 2 button images into the secondary navigation as links to external sites. I know how to add images into Custom CSS Editor and use them in code blocks, however, it's not working in the navigation.

I already have code injection for a sticky Primary Navigation. Is that code stopping me from other codes? I'm fairly new to coding so any help is appreciated.

Site is: https://www.lubnaulaw.com

Creating Nav Link, then use CSS background-image to insert image.

1 hour ago, tuanphan said:

Creating Nav Link, then use CSS background-image to insert image.

I'm not the best at CSS but this is the background-mage code I know... what did I miss?

pay invoice {

  background: url(https://static1.squarespace.com/static/5bfe0f7e3917eebf75a97baa/t/5de8507d0036db4b01b4f9ad/1575506045905/LP_PayRetainer_NO_AMEX_Simple_White.png);

}