Hello, Markus G1
Welcome to Microsoft Community.
If you're using Excel on the web, this shortcut may not be available. Excel on the web has a more limited set of keyboard shortcuts compared to the desktop application. Some shortcuts might not work if they're not supported in the web version, or they might work differently depending on the browser you're using. If you frequently need to use this feature, you might find it more convenient to use the Excel desktop application. If that's not an option, you can manually click on the cell references in the formula bar to navigate to the referenced cells.
Ctrl+[ is a shortcut key used in Excel to trace cell references. For Excel desktop version, you can use the built-in option in Excel called "Trace Dependents" to accomplish this task instead. Select the cell that contains the formula for which you want to find precedent cells. To display a tracer arrow to each cell that directly provides data to the active cell, on the Formulas tab, in the Formula Auditing group, click Trace Precedents ****
. Details about this option can be found here:
Display the relationships between formulas and cells - Microsoft Support
Yuhao Li
Microsoft Community Technical Support