"We can't sign in to your account" window appears.

If you have successfully peered the VPCs and connected the AD, the issue with the "We can't sign in to your account" error message could be related to the following:

Network connectivity: Make sure that the security groups, network ACLs, and routing tables are configured correctly for the VPC peering connection. Check if the necessary ports are open for the AD communication and that the network settings are correctly configured in NimbleStudio.

AD configuration: Check if the AD configuration is correctly set up, including the trust relationship between the AD domains in the two VPCs, DNS resolution, and user permissions. Make sure that the AD users have the necessary permissions to access the NimbleStudio workstations.

Workstation configuration: Verify that the NimbleStudio workstations are configured correctly and have the necessary permissions to access the AD. Make sure that the correct user credentials are entered during the workstation login.

NimbleStudio configuration: Check the NimbleStudio configuration, including the user roles and permissions, and ensure that the NimbleStudio instance is set up correctly to connect to the AD.

If you have checked the above and are still experiencing issues, it may be helpful to review the NimbleStudio documentation and contact AWS Support for further assistance.