This error message referring to the payment reference in the booking screen.

If you are not using this field, you can delete the data that is entered in this field (see image below)

Before you can use the payment reference field, this will need to be enabled first in Twinfield. You can find this setting when you go to General > Company > Transaction types and then you can select purchase invoice / sales invoice.
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The payment reference is indicated in Free Text Field 2 in Twinfield. To enable this, the free text field 2 must be set to allowed like the image below. The header and text must also be selected.

If the payment reference entered in Basecone is also used as part of the  payment description, an additional setting needs to be set in Twinfield. You can do this by going to Cash & Banks > Formats, select the pay format code. Then you can select free text field 2 as a payment description and click on save.

If you do not make use of the payment reference field in Basecone, you can hide this field. In Basecone, go to Settings - Company - Action (edit) - Settings (tab) and go to the option: Enable payment reference field for purchase invoices and change the settings from Yes to No.