Sometimes plugins (and themes) will not adhere to WordPress coding standards and cause compatibility issues. The most common issue is when a plugin incorrectly uses standard/popular jQuery library, not enqueuing the scripts or using old versions.
What to Look For
As you can see you're not able to add any line items, including pre-defined line items. If you're experiencing this issue please let us know via support.
Here's a list of issues that we know about and are working with the respective developers to fix.
Advanced Custom Fields Pro
Issue: Experience shown above.
Resolution: Any fields created for invoices/estimates/clients should not have the "Stylised UI" option enabled.