It's quite sometime for me I worked on SSRS but AFAIR, SUM is an aggregate function and would need a group or data region defined.
Expression Examples (Report Builder) - SQL Server Reporting Services (SSRS) | Microsoft Docs
Quote:The Sum function can total the values in a group or data region. This function can be useful in the header or footer of a group.
Try placing your expression in a group header and see. You have to play with it a little and see how it behaves and work. Would be difficult to exactly help with design part of report online.