I've clicked the Customize button on the invoice screen, and it's working, @ksapexllc. Rest assured that I have additional steps you can perform to apply the correct custom template to your invoices. 

There may be outdated files on your browser that have caused performance issues in QuickBooks Online (QBO). That's why the Customize button isn't working on your end. 

I'd first recommend signing into your QBO account through a private browser. It doesn't store data in the cache. Thus, it's a great way to isolate browser-related issues. If you're using Mozilla Firefox or Microsoft Edge, press CTRL + Shift + P on your keyboard. For Google Chrome, hit CTRL + Shift + N. Press Command + Shift + N if you're using Safari. 

Once logged in, go back to the Invoice screen. Then, choose the appropriate template in the Customize button as seen in the screenshot below. If it works, go back to your default browser. Then, clear its cache. This will help us remove junk files to prevent the unusual behavior on the browser. However, if the issue persists, please use a different browser. 

Additionally, you can go back to the Custom form styles page. It helps you easily manage all your templates in one place. From there, you can preview your existing template as PDF, edit, rename, delete, or make it default. The screenshot below serves as your visual guide. 

Please know that I'm here anytime you have other concerns. Take care always, @ksapexllc.