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Check the API Key (or create the new one) from console.custom.MT and paste it into the field. For more detailed information, refer to this article Credentials .
The next option to try is the correct installation of the plugin. Make sure you have the correct version of the plugin from the RWS appstore.
Refer to this article RWS Trados Studio on how to install the plugin.
As an addition: before installing the plugin, make sure there are no old plugin files in the following folders.
C:\ProgramData\SDL\SDL Trados Studio\16\Plugins\Packages
C:\ProgramData\SDL\SDL Trados Studio\16\Plugins\Unpacked
GPT
How to use your personal API Key from OpenAI
In the section Credentials next to the GPT version, click the three dots and select Edit. Next, check the box that you want to use your API key and provide it. You can find your API Key from OpenAI at https://platform.openai.com/account/api-keys