Generate Embed is not populating the embed map as the REST API call is generating an internal error.
The MySLP_REST_API is expecting the SLP AJAX property to be set and it is null.
Related Call Stack
class.myslp.rest.api.php:438, MySLP_REST_API->get_options() class.myslp.rest.api.php:279, MySLP_REST_API->get_map_options() MySLP.php:257, MySLP->rest_dispatch_request_filter() class-wp-hook.php:324, WP_Hook->apply_filters() plugin.php:205, apply_filters() class-wp-rest-server.php:1187, WP_REST_Server->respond_to_request() class-wp-rest-server.php:1041, WP_REST_Server->dispatch() class-wp-rest-server.php:431, WP_REST_Server->serve_request() rest-api.php:424, rest_api_loaded() class-wp-hook.php:324, WP_Hook->apply_filters() class-wp-hook.php:348, WP_Hook->do_action() plugin.php:565, do_action_ref_array() class-wp.php:418, WP->parse_request() class-wp.php:813, WP->main() functions.php:1336, wp() wp-blog-header.php:16, require() index.php:17, {main}()
Dev Notes
Stack tracing…
- On the initial call the store-locator-plus.php MUP is called first an is loading with the initMySLP _jsonp call from the embed script.
- the SLPlus::initialize_after_plugins_loaded() is being called.
- DOING_AJAX is not set, so createobject_AJAX() is not called.
- MySLP_REST_API->get_options() is called, which expects SLP->AJAX to be set, but it is not.
The front-end/locations.js is using jQuery.ajax() but it is calling a REST API url, not an AJAX listener endpoint… from the locations.js call…

Update the MySLP_REST_API->get_options() method to use SLP_Ajax::get_instance() to ensure that object is instantiated before use when not doing an AJAX call.
Use the main MySLP::getUserBlogId() to fetch blog IDs.
Use SLP_Ajax::get_instance() versus the uninitialized slplus->AjaxHandler to get the SLP AJAX instance.
slplus->AJAX (slplus->AJAX_Handler) was not initialized because the call is a REST call not an AJAX call.