You have applied PATCH 3 update to and now the below API URL does not work.
https://<vRLCM-FQDN/IP>/api/swagger-ui.html
VMware Aria Suite Lifecycle 8.18.0
From 8.18.0 patch 3 .The Swagger-UI for APIs can be accessed at
https://<vRLCM-FQDN/IP>/api/swagger-ui/index.html
This is also updated in VMware Aria Suite Lifecycle 8.18 Patch 3 Release Notes