User has checked the option 'Show retainer summary on invoices' and also set a custom 'Message on Invoice' in Global Settings > Billing, but neither is visible on the invoices.
Core displays the retainer summary for only specific Hourly and Fixed Invoice templates, so you should make sure that you only use those templates in order to display the retainer summary. You can check the preview and settings of various invoice templates from Global Settings.
The custom Message on Invoice that you set at the global level only applies to the new clients and projects that you create. If you want to set this for invoices of existing clients and projects, you need to set it at the client level (Client Details > General > Billing Options > Invoice Options) or project level (Project Details > General > Billing Options > Templates). The message set at the project level takes precedence.