This error message is familiar ? We will explain one of the possible rootcause and how to fix it...

The NULL result might be caused by the absence of the quantity field for some products.
Looking at the Prestashop's DB fields in the product_qty table, we see that they were typed as DECIMAL instead of INTEGER.

This has the effect of storing strange values like 0.5, 1.0, 1.5,...

In case it's 0.5, it's rounded to zero which leads of blocking the printing labels and giving the message "Error in request null"

In order to eliminate such errors, it would be necessary to return the quantity field in its original format: INTEGER.