&1 is not a valid value; valid values are 0...5

What causes this issue?

In cash deconcentration, optimization can only be planned a maximum of
five days in advance. Valid values for the "number of working days" are
therefore in the areas from 0 to 5. Optimization only occurs for the
current day if the value 0 is entered.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Enter a value from area 0 to 5.

Error message extract from SAP system. Copyright SAP SE.