Dave, are you still engaged with Zen Cart? There are a bunch of issues with the new version as have been noted here; not sure if we need to recommend the use of the prior version or not.
Here are a couple of logs:
[22-Jul-2022 14:53:01 America/New_York] Request URI: /mystore/newsletter.html, IP address: ::1
#1 require() called at [/Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_main_page.php:202]
#2 require(/Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_main_page.php) called at [/Users/scott/sites/mystore/index.php:94]
--> PHP Warning: require(includes/templates/template_default/templates/tpl_newsletter_default.php): failed to open stream: No such file or directory in /Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_main_page.php on line 202.
[22-Jul-2022 14:53:01 America/New_York] PHP Fatal error: require(): Failed opening required 'includes/templates/template_default/templates/tpl_newsletter_default.php' (include_path='.:/Applications/MAMP/bin/php/php7.4.21/lib/php') in /Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_main_page.php on line 202
[22-Jul-2022 14:36:18 America/New_York] Request URI: /mystore/, IP address: ::1
#1 require() called at [/Users/scott/sites/mystore/includes/modules/bootstrap/subscribe_header.php:20]
#2 include(/Users/scott/sites/mystore/includes/modules/bootstrap/subscribe_header.php) called at [/Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_header.php:149]
#3 require(/Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_header.php) called at [/Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_main_page.php:124]
#4 require(/Users/scott/sites/mystore/includes/templates/bootstrap/common/tpl_main_page.php) called at [/Users/scott/sites/mystore/index.php:94]
--> PHP Warning: require(includes/templates/template_default/templates/tpl_subscribe_header.php): failed to open stream: No such file or directory in /Users/scott/sites/mystore/includes/modules/bootstrap/subscribe_header.php on line 20.
[22-Jul-2022 14:36:19 America/New_York] PHP Fatal error: require(): Failed opening required 'includes/templates/template_default/templates/tpl_subscribe_header.php' (include_path='.:/Applications/MAMP/bin/php/php7.4.21/lib/php') in /Users/scott/sites/mystore/includes/modules/bootstrap/subscribe_header.php on line 20
Also the images in admin are not found when running the subscription_manager.php page.