Universal Containers (UC) uses a custom object called Vendor. The Vendor custom object has a Master-Detail relationship with the standard Account object. Based on some internal discussions, the UC administrator tried to change the Master-Detail relationship to a Lookup relationship but was not able to do so. What is a possible reason that this change was not permitted

The account record comprises of Invoice roll up summary fields.

Explanation:

The possible reason that this change was not permitted was that the account record comprises of Invoice roll up summary fields.

Invoice roll up summary fields: A roll-up summary field computes values from associated records, for example those in a linked list. a person or someone ca design a roll-up summary field to show a value in a master record by building  the values of fields in a particular record.

The detail record must be associated to the master through a master-detail relationship. for instance, you want to show the sum of invoice amounts for all linked custom object records in an account’s Invoices related list. you can show the whole or sum in a custom account field refereed to as Total Invoice Amount.

answer; correction;

well you could find somebody who doesn't want theirs and ask for it or possibly trick a dealer into giving it to you for free.