Pixel Errors

Pixel not paired with a Catalog
Don’t worry if you don’t want to run Dynamic Product Ads (DPA). If you do, what’s needed is to create a catalog & upload products to it through a data feed. Then, connect a pixel to that Catalog as a data source. Check out how to set up your catalog and start retargeting here.
Pixel firing 2 times on the page
Chances are you have accidentally set the pixel to fire via another tool, such as Facebook Sales Channel (the Shopify app), Google Tag Manager, your theme, etc.
Check this out to solve this.
📝Good to know: There are even more common errors if you are using Conversion API events (which I am sure you would like to). But most of them are solvable or false-positive warnings. That means you don't need to worry too much.