Deploying a cloud-hosted environment through LCS requires the LCS administrator to specify an environment name. Limited to 20 characters, this name must be unique across an Azure region, rather than within a tenant or subscription. There's no indication in the deployment wizard or the documentation - https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/dev-tools/access-instances - to this effect, and whilst the error message "Lifecycle Services tried to deploy the environment, but there's already an existing environment that has the same domain name. Please try to deploy the environment again. When deploying, enter a different name for the environment. If the issue persists, please contact support with this Id: %session id%" does suggest changing the name, the explanation could be more comprehensive (or could point to more comprehensive documentation), particularly given that the phrasing "if the issue persists, please contact support" implies that this may not be expected behaviour with known constraints.