I first thought that the recent calculation group feature wasn’t working in a matrix when selecting more than one calculation item from a slicer, until, I realised that all I had to do to fix it was to simply disable the “column subtotals” option.
To be honest I find the error message shown by PBI misleading.

So if you encounter the error”Time Calculation Requires selection of exactly one calculation item” here is what you have to do to fix it: (video at the bottom)

1- Make sure to clear your slicer selection

2- Click on the matrix visual and head to the format menu (the paint roller)

3- In the search bar, type “column subtotals” or scroll down to the subtotal menu.

4- Disable the column subtotals option

5- Head back to the slicer and that’s it now we can select multiple Calculation Items!

Watch the full mini tutorial here: