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Hi @thuelsey1981,
As the error states, it’s related with a class that doesn’t exist. It’s likely that you’re using a code snippet that was calling a class from a plugin you have deactivated or uninstalled. Search within your code snippets, and deactivate the code snippet that is using this class, then try generating a PDF invoice to check if it opens correctly.
By the way, after a quick search, I realized that this class is from Advanced Shipment Tracking for WooCommerce. You can see a related topic that may be help here: