The steps mentioned below make the SLA selection mandatory for any service request creation.
Tested in builds : 14000 and 14301.
Script:
if(!jQuery("#servicesla-selected").attr("data-association-id") || jQuery("#servicesla-selected").attr("data-association-id") === '0' ){
alert("Kindly Choose a SLA to proceed");
$CS.stopFormSubmission();
}
Field and Form Rule Configuration:
Under Admin > Field and Form Rules > Service > On Form Submit
Whenever a service request is created using the mentioned templates, upon submitting the form, a pop-up will appear, prompting us to select an SLA and making it mandatory.

Note: If the "All templates" option is selected, any templates that do not have an associated SLA will be blocked from creation or editing. Therefore, it is advisable to set templates that have an associated SLA.