Welcome to the Community. I'd be glad to assist you with adding a new customer.
When QuickBooks Online exhibits unexpected behavior, it's good to begin with some general browser troubleshooting. This can help determine if the cause of this is related to the browser that's being used.
The first step that's generally recommended is to clear the cache that's stored in the browser. Cache and temporary internet files are stored in order to increase loading performance, but too much can accumulate and interfere with processes. For additional information on this, as well as steps for supported browsers, please visit the following link: Delete or Disable Cache and Temporary Internet Files in Your Web Browser.
Should you not be able to add a new customer after clearing cache, I suggest trying an alternative browser. This can further help to isolate the cause of being unable to add the customer.
I'm confident that this information will help you get back to business.
Don't hesitate to let me know if I can be of further assistance. Take care.