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Support for regions in the AWS GovCloud (US) partition is an opt-in Labs Feature:
The AWS GovCloud (US) partition provides the following regions: AWS GovCloud (US-East) – us-gov-east-1,
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AWS GovCloud (US) – us-gov-west-1
This partition requires dedicated credentials, see How do Government agencies, contractors and customers access the AWS GovCloud (US) Region?: "Customers cannot sign up for AWS GovCloud (US) through the traditional, online AWS sign up process. AWS must engage with the customer directly to sign an agreement specific to the AWS GovCloud (US) Region. [...]"
Due to being a non US company, we are not currently in the position to test this app with the AWS GovCloud (US) Region directly. However, the API is compatible in general and we have done our best to address the documented differences - please get in touch if things do not work as intended, we are very interested to collaborate on the necessary adjustments.