This article explains why the Add Row-Level Formula option is unavailable in reports created with "with or without" Custom Report Types. A workaround using custom formula fields is provided.
The Add Row-Level Formula menu is not available in reports based on Custom Report Types configured with the "with or without" relationship option (e.g., " 'A' records may or may not have related 'B' records").
As a workaround, you can achieve similar calculations by creating a Formula Field directly on the object. Navigate to Setup, open Object Manager, select the target object, click Fields and Relationships, and create a new Formula field with the desired calculation.
Get the Most Out of Row-Level Formulas: Tips, Limits, and Limitations
Add Row Level Formula option is not available
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