Receiving this error in a WPEngine hosting environment. We do not know what triggered it but we did see that sometimes $this->nexus can be null in this function in certain instances. Added a simply null check:
Error Details
=============
An error of type E_ERROR was caused in line 24 of the file /nas/content/live/oursite/wp-content/plugins/taxjar-simplified-taxes-for-woocommerce/includes/class-wc-taxjar-nexus.php. Error message: Uncaught TypeError: count(): Argument taxjar#1 ($value) must be of type Countable|array, null given in /nas/content/live/oursite/wp-content/plugins/taxjar-simplified-taxes-for-woocommerce/includes/class-wc-taxjar-nexus.php:24
Stack trace:
#0 /nas/content/live/oursite/wp-content/plugins/taxjar-simplified-taxes-for-woocommerce/includes/class-taxjar-settings.php(330): WC_Taxjar_Nexus->get_form_settings_field()
taxjar#1 /nas/content/live/oursite/wp-content/plugins/taxjar-simplified-taxes-for-woocommerce/includes/class-taxjar-settings.php(208): TaxJar_Settings::get_nexus_settings_display()
taxjar#2 /nas/content/live/oursite/wp-content/plugins/taxjar-simplified-taxes-for-woocommerce/includes/class-taxjar-settings.php(645): TaxJar_Settings::get_settings()
taxjar#3 /nas/content/live/oursite/wp-includes/class-wp-hook.php(310): TaxJar_Settings::save('')
taxjar#4 /nas/content/live/oursite/wp-includes/class-wp-hook.php(334): WP_Hook->apply_filters('', Array)
taxjar#5 /nas/content/live/oursite/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
taxjar#6 /nas/content/live/oursite/wp-content/plugins/woocommerce/includes/admin/class-wc-admin-settings.php(77): do_action('woocommerce_set...')
taxjar#7 /nas/content/live/oursite/wp-content/plugins/woocommerce/includes/admin/class-wc-admin-menus.php(153): WC_Admin_Settings::save()
taxjar#8 /nas/content/live/oursite/wp-includes/class-wp-hook.php(310): WC_Admin_Menus->save_settings('')
taxjar#9 /nas/content/live/oursite/wp-includes/class-wp-hook.php(334): WP_Hook->apply_filters(NULL, Array)
taxjar#10 /nas/content/live/oursite/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
taxjar#11 /nas/content/live/oursite/wp-settings.php(654): do_action('wp_loaded')
taxjar#12 /nas/content/live/oursite/wp-config.php(121): require_once('/nas/content/li...')
taxjar#13 /nas/content/live/oursite/wp-load.php(50): require_once('/nas/content/li...')
taxjar#14 /nas/content/live/oursite/wp-admin/admin.php(34): require_once('/nas/content/li...')
taxjar#15 {main}
thrown