First, you need to make sure you are printing 4x6 inch/ 150x100mm format label.
Most platforms generate label of 8.5x11 inch, you need to change it to 4x6 inch, then download pdf file and print.
For some platform may not provide 4x6 inch label, you need to use adobe acrobat reader DC "Snapshot" function to crop 8.5x11 inch label.
Here download set up guide for some popular platforms: https://cdn.shopify.com/s/files/1/0657/6626/0980/files/Platform_Setup_Guide.pdf?v=1676530583
After downloading 4x6 inch labels, you need to set the paper size as "4x6 inch / 150x100 mm", then select "Fit to paper" and print.