Use at least the defined length &

What causes this issue?

You are editing an input/output field on the screen and have entered a
defined length of less than the minimum possible length.
If the input/output field has no icon, you have entered 0 as the
defined length. This would correspond to an empty field.
If the input/output field has an icon, you have specified a length that
is insufficient for the system to display the icon. Input/Output fields
with icons use the first four characters to identify the icon, and the
fifth to separate the icon from the subsequent output template.

System Response

The system cannot accept this value for the defined length.

How to fix this error?

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

Error message extract from SAP system. Copyright SAP SE.