You can't freeze panes in Numbers the way you can in Excel.
In general you don't need an Excel-like freeze panes functionality because the Numbers design favors multiple special purpose tables on a sheet rather than one big expanse of cells.
In Numbers you can define up to five columns on the left as Header Columns and freeze those. You can also define up to five rows at the top as Header Rows and freeze those. See the Add, Remove or freeze header rows and columns section here:
Add or remove rows and columns in Numbers on Mac - Apple Support
If you're feeling the need for freeze panes Excel-style, then you might consider breaking up your work into smaller tables.
If you haven't done so already have a look at the templates at File > New in your menu. They are good examples of efficient document design in Numbers.
SG