The most simple calculated fields return #Error when placed on a Report Builder report.
For example, following the instructions at https://learn.microsoft.com/en-us/sql/reporting-services/report-data/add-edit-refresh-fields-in-the-report-data-pane-report-builder-and-ssrs?view=sql-server-ver16, I created a calculated field: MyCalcExpr = "Hello".
I place the calculated field on my report in Report Builder. When I run the report, MyCalcExpr shows #Error.
I am using Microsoft Report Builder 15.0.20073.0. Report Builder on-the-whole is fairly flakey and can be difficult to use. A good deal of this is due to confusing terminology. For example, I know what a parent and child are in SQL and entity relationships; but the meaning of terms such and "parent group" and "child group" is unclear. The documentation does not really help here. Simple concepts such as a footer line that fires when a group changes can be difficult to figure out in report builder. The software tries to "help" when working with groups. To obtain the report that you really want, you must often undo most if not all of the additions made for you in report builder. Things that I can do in minutes in Crystal Reports or even Access, are difficult slogs in report builder. Report builder users could really benefit from a "Don't help me" button.
Terminology matters. Think of all of the books on Git that have been published. How many of these would be necessary if Git had been developed with intuitive and descriptive syntax. The learning curve for Git would be shallow if the developers of Git had applied the Simplified Technical English Standard to create an intuitive command set. Instead we have Git documentation that describes one vague command in terms of several other ambiguous commands. Git may be a standard, but it is a needlessly painful and expensive one.
When it comes to terms such as "Parent Group", "Child Group", "Adjacent Before", "Adjacent After", "Recursive parent", and others; Report Builder suffers from the same issues. This is compounded by options such as "Code". Is the VBA or is this .NET?
The interface is out-of-date and shows signs of age. I don't know if Report Builder has a roadmap into the future or will be kicked to the open-source curb and abandoned.
Using Report Builder is confusing and frustrating experience. Many things just don't work or work in vague and unexpected ways.