Submit button / Order now button on catalog items is missing in Service Portal for some users.
The user has snc_read_only role added. The OOTB catalog item widget shows the submit/order now button only to the users who do not have the snc_read_only role.
The below Line in the server-script of the widget (ID: widget-sc-cat-item-v2) does the role check.
data.can_create_cart_item = canCreateCartItem(className) && !gs.hasRole('snc_read_only');
Remove snc_read_only role from the user profile.