If you are on-premise or DT you have access to the Language Maintenance UI which allows you to adjust the message strings presented to your users. There are two utilities for adjusting the text - one that works specifically for the static labels and text presented on each UI and one that allows you to adjust error messages originating in code.
Sounds like the string you are attempting to adjust is a code generated message and to change that text you would use the the Language Maintenance UI found under the System Setup > System Maintenance menu. The Utility is not entirely straight forward but the steps to do what you want are:
Run Language Maintenance UI
Enter ENU for language ID
Click on “Translations” Sheet tab
Click on the “Get Translations” button - Black button under sheet tabs
Click on the “List” Sheet tab under the Get Translations button
You will be presented with a list of translations and three filter options. Use the “Original Text” filter option to find the Message you want to adjust - you only need to enter the part of the text that makes it unique or more unique.
Select the Line with the Message you want to update. You can now adjust the text either in the grid or on the “Detall” sheet (first one). You need to leave the Original Text alone and update the “Translated Text” column with the verbiage you want to use instead of the Original Text.
When updating the Translated Text, be very careful not to remove any substitution references like {0} or ‘{1}’. You can move them around in the string but you should maintain the same substitutions as found in the Original Text.