What attribute would cause a runtime error in a Customizable Rollup?

A runtime error in a Customizable Rollup can occur when attempting to roll up data to a target field that has an active validation rule. When a validation rule is triggered, Salesforce prevents the record from being saved if the data being inserted or updated does not meet the rule's criteria. In the context of Customizable Rollups, if the target field is subject to such a validation rule, it could not accept the values the rollup is attempting to input, thus resulting in a runtime error.

The other options pertain to different scenarios that do not inherently lead to runtime errors within the process of rollup. For example, custom objects and NPSP objects can still participate in rollups, and as long as the target field corresponds to the appropriate object, no immediate issues arise from those choices. However, the presence of an active validation rule distinctly disrupts the operation of the rollup, which is why it is identified as the cause of a runtime error.