It's nice to see you in the Community. I'm here to lend a hand with running a Budget vs. Actual report.
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Generally, the option to customize a report is going to be in the top right-hand corner of the screen. Just to confirm, have you noticed this option missing on any other reports? I've included a screenshot as a visual reference:
Another step that would be helpful to determine why the option is missing would be to cycle to a different budget, if you have one available, and see if the Customize option appears. Here's how:
Select Reports on the left navigation menu.
Enter Budget vs. Actuals in the Report Search Bar.
Click the drop-down arrow under Budget and pick an alternative budget.
Select Run Report.
I've included a short video to serve as a guide:
I want to ensure that this is taken care of for you, so please let me know how it goes by posting a reply. Hoping this response finds you with a smile.