Conditional formatting greyed out in Excel is typically as a result of the workbook being a shared workbook.
To check whether you have the shared workbook feature switched on, go to the REVIEW tab and click the SHARE WORKBOOK button. If the ‘Allow changes by more than one user…’ is ticked then the workbook is shared and you can use conditional formatting.
Note however that this does not mean you can’t have it work on a Shared Workbook. All you need to do is establish the conditional formatting while the workbook is in normal mode and only change to shared mode after you are happy with all the rules.
If you want to edit the conditional formatting you will also need to switch off sharing mode, make the changes, and then switch sharing mode on again.
Related: If you are interested in making your Excel spreadsheets more visually appealing perhaps look at our Excel Dashboards course.