The list of the fields that can be selected via the Fields "wizard" on the right-hand pane are those that have simple "translations" to a text column, such as text or numeric columns in your SharePoint list, or even choice columns with a single selection. Since Category is a multi-select choice column, then the conversion is not as trivial, so the wizard doesn't offer that option.
You can update it yourself, however. For example, you can select one of the existing labels in the gallery (or add a new one), and update its Text property with a function such as Concat that can combine multiple elements into a single one, like in the example below:
Concat(ThisItem.Category, Value, ", ")
Here's an example; I have a SP list with a few fields:
If I add a gallery and try to use the "Fields" dialog to choose a multi-selection choices field, it won't show up (as I mentioned above):
So to use that we need to edit the expression for the label in the formula bar directly. In this example I'm replacing the 'ChoiceSingle' field, but you could also add another label to hold the multi-selection field:
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