Unable to generate PDF

I have installed the latest version (2.2.7) of this plugin onto our staging site.

When I go to generate a label I get the following error.

What could be wrong?

Notice: Trying to get property of non-object in /nas/content/staging/camleyphoto/wp-content/plugins/code-snippets/php/snippet-ops.php(517) : eval()’d code on line 3

Notice: Trying to get property of non-object in /nas/content/staging/camleyphoto/wp-content/plugins/wp-lister-ebay/classes/core/WPL_Functions.php on line 78

Fatal error: Uncaught exception ‘Exception’ with message ‘FPDF error: Some data has already been output, can’t send PDF file’ in /nas/content/staging/camleyphoto/wp-content/plugins/shipping-labels-for-woo/wf-template/pdf-templates/fpdf.php:271 Stack trace: #0 /nas/content/staging/camleyphoto/wp-content/plugins/shipping-labels-for-woo/wf-template/pdf-templates/fpdf.php(1063): FPDF->Error(‘Some data has a…’) #1 /nas/content/staging/camleyphoto/wp-content/plugins/shipping-labels-for-woo/wf-template/pdf-templates/fpdf.php(1012): FPDF->_checkoutput() #2 /nas/content/staging/camleyphoto/wp-content/plugins/shipping-labels-for-woo/wf-woocommerce-packing-list.php(254): FPDF->Output(‘#90089-Shipping…’, ‘D’) #3 [internal function]: Wf_WooCommerce_Packing_List_Woo->wf_packinglist_print_window(”) #4 /nas/content/staging/camleyphoto/wp-includes/class-wp-hook.php(286): call_user_func_array(Array, Array) #5 /nas/content/staging/camleyphoto/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array) #6 /nas/content/staging/ in /nas/content/staging/camleyphoto/wp-content/plugins/shipping-labels-for-woo/wf-template/pdf-templates/fpdf.php on line 271