The anxiety in that situation is familiar to anyone who has ever been asked to perform under a watchful eye. The user knows their tools, has real experience with VLOOKUP, pivot tables, and conditional logic, and even feels comfortable in R and SQL. That is a strong foundation. Our plain opinion is this: the assessment is not a test of whether you know Excel. It is a test of how you solve problems under pressure. And the skills you already have, especially your comfort with programming logic, are your greatest advantage.
Excel is not a separate language from what you already speak. The same analytical thinking that makes you effective in R and SQL applies directly to spreadsheet work. When you write an IF statement in Excel, you are doing the same kind of conditional reasoning you use in any coding environment. When you build a pivot table, you are grouping and aggregating data the same way you would with a GROUP BY clause. The difference is the interface, not the logic. That means you are not starting from scratch. You are translating existing knowledge into a different syntax. The pressure of a live camera makes that translation harder, but it does not make the knowledge disappear.
Here is what we would say directly to someone in that position. Prepare for the environment, not just the content. Practice in a setting that mimics the test: open Excel, turn on your webcam, and work through a problem while speaking your thoughts aloud. Talk through your steps as if you were explaining them to a colleague. That habit does two things. It forces you to slow down, and it gives the evaluator a window into your reasoning even if your clicks are not perfect. If you get stuck, name the problem. Say, "I know this requires a lookup, but I need to check whether the table array is structured correctly." That shows competence far more than silence does.
You already have the hard part, the analytical instinct. The assessment is a performance, not a judgment of your worth. Walk into it knowing that your R and SQL experience has already taught you how to think about data. Excel is just another way to express that thinking. Keep your hands on the keyboard, your voice in the room, and your focus on the problem in front of you. That is how you move from anxious to effective.