There can be various reasons why this is happening.

If you purchased a plane directly from the manufacturer you have to wait until it is delivered to you. You can check the delivery dates on the fleet page as well as the manufacturers page.

If you are using the semi-automatic scheduler (the one to schedule to a specific route), the dropdown list of aircraft only shows aircraft that are available at the selected departure time. It is possible it doesn't show your airliner because it's already flying a different flight at the time.

The 'Quick assign' or auto scheduler is still in development and doesn't always behave as intended.

If it's still something different that's going on, can you send me some more information? (including images or even a save file + an explanation of what you did)