Hi @Harini ,
How do you create your custom connector? Through "Import an OpenAPI file" or "Import an OpenAPI from URL"?
Based on the screenshot that you mentioned, I think there is something wrong with your imported OpenAPI definition file.
As an alternative solution, please consider create your custom connector from Blank as below:

Then specify the Host and Base Url for your custom connector as below:

Then you may need to specify a proper authentication way for your custom connector, if this Rest API you want to access do not need any authentication, please just specify "No authentication" option within the Security Tab.
Within the Definition tab, click "New action" option to add action path within your custom connector. Configure it as below:

Then you could edit the "User_Key" query parameter and set a default value for it as below:


After that, you need to specify a proper response Body for this action path. Just click the "Default" response, then click "Import from sample" option, then type proper JSON Schema within the Body property:

Note: In order to get proper response for this HTTP request, you could test this request using Postman Tool, then copy the Response result into above Body property.
More details about creating a custom connector in PowerApps, please check and see if the following video could help in your scenario:
https://www.youtube.com/watch?v=dBCS1nPsDiE&t=1s
Best regards,