Hi,
The following text mode would do it:
displayname=Days on Hold
linkedname=project
namekey=view.relatedcolumn
namekeyargkey.0=project
namekeyargkey.1=Days Project Has Been On Hold
querysort=DE:project:Days Project Has Been On Hold
textmode=true
valueexpression=SUM({project}.{DE:Days Project Has Been On Hold},1)
valueformat=customNumberAsString
NOTE that the value format is "customNumberAsString" in my code, however in your original code it was customDataLabelsAsString. This suggests that the field has been set up as text format instead of number format. You're likely to find that mathematical expressions will not work with the field set at text.
Best Regards,
Rich.