Error – Addon unusable?

I am trying to get this to work, but I get the following error as soon as I submit the form. Deactivating the Srout process rule will submit the form, but then the addon is not used, of course.

Any suggestions?
Fatal error: Uncaught Error: Call to undefined method SI_Estimate_Submissions::__() in /home/stoneh1q/public_html/wp-content/plugins/caldera-forms-sprout-invoices-integration/cf-sprout-invoice.php:188 Stack trace: #0 /home/stoneh1q/public_html/wp-content/plugins/caldera-forms-sprout-invoices-integration/cf-sprout-invoice.php(119): cf_si_maybe_create_estimate(Array, 'fp_46424020') #1 /home/stoneh1q/public_html/wp-content/plugins/caldera-forms/classes/core.php(3192): cf_si_process_form_estimate(Array, Array, '_cf_process_5a9...') #2 /home/stoneh1q/public_html/wp-content/plugins/caldera-forms/classes/core.php(4882): Caldera_Forms::process_submission() #3 /home/stoneh1q/public_html/wp-content/plugins/caldera-forms/classes/core.php(3572): Caldera_Forms::process_form_via_post() #4 /home/stoneh1q/public_html/wp-includes/class-wp-hook.php(286): Caldera_Forms->cf_init_system('') #5 /home/stoneh1q/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #6 /home/stoneh1q/public_html/wp-includes/plugin.php in /home/stoneh1q/public_html/wp-content/plugins/caldera-forms-sprout-invoices-integration/cf-sprout-invoice.php on line 188