Landing Page Error: Not working in test mode - Help

Hello Experts,

I am on SAP Marketing On Prem 1709 version. I have created a basic landing page with email address and one subscription category for testing purpose.

I have test the landing page in test mode( standard feature for sap marketing). when i click on submit button with valid email address then i am getting the above error?

Can anyone assist what can be the issue. ? I have created similar landing pages for cloud system and there is no issue.

I think in test mode it should work for prem version as well.

Note: This error is coming in test client. For production if i create a test page and check then i am not getting an error.

Can someone also let me know if i can debug the issue?