My file has over 1000 lines by sku & customr, I need to add line
between each sku group. This is a data file, I don't want to pivot
table to add line, instead of manually add line between each sku, does
excel have a formular to add line?

This isn't a formula, but conditional formatting might help.

Select the whole sheet.

Then
Format Conditional formatting

In the Condition 1 dialog box,
Choose "Formula is" in the pull-down list
The following formula (assuming column A has the SKU value):
=$A1<$A2
Click "Format" and choose a single underline only

Modify to suit.