The Render button in the Render As window will remain grayed out until you select a rendering template. To make the button available, follow these steps:

The Render As window contains a range of formats to choose from. Start by selecting a format.

Each format provides multiple templates. Select one of the templates. To see the templates for the selected format, click the small black arrow (marked in red in the image).

When you select a template, a file extension will be added to the name field, and the Render button will become available.
Click the Render button to start the rendering process.