It can take a while for newly created or newly shared environments to show up for users. The Microsoft guidance says it may take up to 10 minutes for them to become available in the Power Platform developer studios and the Power Platform Admin Center. However, on occasions I have known it take a lot longer. This can be frustrating when waiting to get access.
Additionally, Microsoft known issue 423742 active since 25th July, is preventing some newly created environments from being viewable for selection in the Power Platform studios. I created some new environments 4 days ago and they were still not available, so this can happen even if you are the one who created the environment!
While the environment owner can see the new environment in the Power Platform Admin Center, this issue prevents other users who have been given admin rights from doing the same.
Here is the recommended workaround so environments can be accessed almost immediately by you, or users you have given access rights to.
First, obtain the Environment ID which is a GUID. The owner of the environment can obtain this in the Power Platform Admin Center under Environments and selecting the required environment.
Once you have the Environment ID, use it with the URLs below, replacing GUID with your Environment ID, to access the environment:
Power Apps Studio
https://make.powerapps.com/environments/GUID/home
Power Automate Studio
https://make.powerautomate.com/environments/GUID/home
Power Pages Studio
https://make.powerpages.microsoft.com/environments/GUID/portals/home
Power Platform Admin Center
https://admin.powerplatform.microsoft.com/environments/environment/GUID/hub
Summary
This will get around the current service issue and give you access to the environment much quicker than waiting for it to become available to select. If you are an Admin, once users have been given environment access by assigning them an appropriate security role, send them the link.
When an environment has been viewed using the above URLs, it usually triggers the environments to become accessible directly from the Power Platform studios or Admin Centre, so you only need to do it once.
Here is the Microsoft troubleshooting page on missing environments.
More tips on managing Power Platform Environments: Power Platform Environments Demystified