How to substitute the same variable with hundreds of different values in different rows

the excel is not a part of issue you need help with, so is irrelevant to your posting.
the result of reading the excel with read_xlsx is critical. i.e. the df object you made.

Sharing that (df), or part of that, or a facsimile of that, would make a reprex. A reprex will help others help you.

Here is a link to the guide.