@Eline Bijkerk: my Microflow looks like this:
So you have SUB_CreateExcelExport, but you are not returning a value there. This means that it isn't available in the microflow that you posted a screenshot of.
To fix this issue, go back into the SUB_CreateExcelExport flow, and select the correct return value. If you're not sure how to do that, scroll up a bit in the learning path – there is a screenshot there showing you what your flow should look like :)