Market Place application (solution template)

I try to create a Azure Market Place application (solution template) and try to test the solution near my Subscription with ARM template deployment :
1. When I used resources i was able to create\select vnet and create subnets inside if the virtual network was under the same RG of the main ARM.
2. the issue started when i tried to select existing vnet on a different RG (that required me to change the resources to be "Microsoft.Resources/deployments" instead of resources)
 
I modified the deployment to get a new parameter of the vnet RG and passed it to the deployment and now I'm receiving a validation error when I try to deploy (using custom deployment from a template in the portal).
the error received from the validation step is:

Deployment template validation failed: 'The resource 'Microsoft.Network/virtualNetworks/test1/subnets/test-in' is not defined in the template. Please see https://aka.ms/arm-syntax for usage details.'. (Code: InvalidTemplate)

Thank you.