Do you use the Hierarchy Slicer custom visual which is downloaded from Visuals Gallery? If that is the case, based on my test, you can enable Multi selection via the formatting pane as follows, just turn off Single select. However, it is not possible to press Ctrl to select multiple fields in the slicer.
In addition, if you have any other issues about the Hierarchy Slicer custom visual, you can directly contact the author. In this link, you can send an email to the author by clicking “Contact Author”.
Community Support Team _ Lydia Zhang
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.