Sharepoint - Page Library - new column with required field, how to prevent page creation if required field not filled

This is by design in SharePoint Online Modern experience.

The list items/library files with incomplete/missing information will be shown in separate view called Items/Files that need attention like below:

Also it will show small message of Required info in column in list view (see in above image).

Workarounds:

1. Set Default value to your "Content Type" column from column settings.
2. Use Approvals in SharePoint - You can approve the site pages only after all information is filled for the newly created pages.

Update:

Setting default value to column:

1. Go to Site Pages --> Library settings.
2. Navigate to Columns section and click on the column name "Content Type".
3. Scroll down to Default Value and set one of the choices in textbox below it.
4. Click OK.

Default Value Set automatically to Newly created Page:

In Site Pages Library: