I hope you have selected the allowed controls in the placeholder you added. The steps to add a placeholder and then the allowed controls are as below:
1. Go to Layouts -> Placeholder Settings
2. In case you have not already added your placeholders in the Placeholder Settings, right click on it and select insert Placeholder. For example: "main" (If already added, please skip this step.)
3. Now, select the placeholder you have created and add your controls / components (e.g. Header, etc) in the Allowed Controls field. Please refer the screenshot shown below:
Hope this helps.