Cannot select customer when creating an invoice. A dropdown menu normally shows me options but nothing. Products/services don't show up either- I can't do it manually.

I can think of a reason why fields aren't populating while creating an invoice in QuickBooks Online, @mtmhts. I'm here to help you isolate this.

I tried entering an invoice, and everything seems to be working properly on my end. Also, there's currently no known issue documented in QBO regarding this. You're likely experiencing a browser-related problem that caused the fields not to populate. Let's perform some basic troubleshooting procedures to rule this out. 

Start by signing in to your QBO company using an incognito window. Here are the shortcut keys to open it:

• For Firefox or Microsoft Edge, press Ctrl + Shift + P.
• For Chrome, press Ctrl + Shift + N.
• For Safari, hold down Command + Shift + N.

If this works, then you'll need to clear your regular browser cache. This prevents unusual browser behavior. 

If the issue continues, use another supported browser.

Once you're good, check out this additional guide on how to personalize invoices in QBO: Customize invoices, estimates, and sales receipts in QuickBooks Online.

Keep in touch if you need more assistance with invoices or QuickBooks. I'm always ready to back you and your business. Keep safe.

Please clear your browser cache , and then sign-in again to quickbooks, as this issue is not a QBO issue, it's a browser cache issue.