I've got you covered, @cantareadio.

This may be a browser-specific issue. To fix this, let's access your QuickBooks Self-Employed account using a private browser. Private mode prevents storing cache and cookies that might cause problems when running websites.

Here are the shortcut keys you can use:

• Google Chrome: press Ctrl + Shift + N 
• Mozilla Firefox and Microsoft Edge: press Ctrl + Shift + P
• Safari: press Command + Shift + N

When signed in, follow the outlined steps provided by my colleague, ShiellaGraceA to change your email greetings. For more info about the process, please visit this article: Create invoices in QuickBooks Self-Employed. 

If this works, let's clear the cache of your regular browser. It will remove the stored cache or cookies that might cause the problem and this also refreshes your browser's background processes. If you still can't see the customize button, use another supported browser. It could be that the one you're currently using has a temporary issue with QuickBooks. 

Then, if in case you want to get a copy of your invoices just go to the Invoices menu and choose print or export as PDFs.

Please know that you can always post here if you have further concerns about changing your email greeting in QBSE. I'll be around whenever you need further assistance. Take care always.