Power BI Dataflows are still pretty new to most. I think it is amazing that Power Query has moved to the cloud and you can use it with a Power BI Pro license only.
Some things are better than in the offline version of Power Query, like new automatic transformation steps or suggestions for transformations.
Some things however are problematic, like time intelligence and restrictions on which data types you can output. Microsoft's documentation is very patchy as well.
It is e.g. currently not possible to have a duration column in your output. This random error message will appear:
After further investigation it seems like there can be multiple time intelligence errors (as of early Jan 2021) with Power BI Dataflows.
If you get cryptic error messages like above make sure to look at your time intelligence.
But what if you really need to use data types like duration?
The good thing is that even if you cannot have it in your output, you can do the following:
This means we can use duration data types as long as we change it to something else before we Save & close.
Like in my example, changing it to decimal (so we can still do calculations), might be a viable fix.