Without seeing your Excel sheet, knowing the style number & brand of label, as well as the specific steps you're taking it's rather difficult to determine. All I can offer is the appropriate procedure.
The list in Excel should have a Field Name at the top with no blank cells in the list, such as:
In Word go to the Mailings tab, then;
- Click the Start Mail Merge tool & select Labels
- Choose the appropriate label Product Number, OK
- Click Select Recipients> Existing List then pick your Excel file
- Click Yes
- Click OK
- Open the Insert Merge Field list to insert the field name on the first label
- Click the Update Labels tool
- Finish & Merge as you wish
See macOS page of this Word Help article on Mail Merge to Labels for more info.