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Number Card creation problem with SUM Aggregation for custom app

The “Document Register” custom document type has a “Currency” field with the required numerical data.
Even so, I can’t create a Number Card with SUM aggregation for custom app because there is no option to select in the “Aggregate function Based On” list field.

Would you please advise where the problem could be?

ERPNext version

Installed Apps
Documents App: v0.0.1 (main)
ERPNext: v13.15.0 (version-13)
Frappe Framework: v13.15.0 (version-13)