You can create approvals via script, but in this case you also want to link the approval to the sc_task record so that it shows on the Related List. Your current script may be creating orphaned approval records, or there could be a Business Rule or Data Policy in place to prevent the insert if not all of the required fields are populated. Here the fields that I have populated when creating approval records via script. I've only ever created approvals at the RITM level - when approval is required before the flow/workflow continues to scripts/Catalog task, etc. but the script should work the same at the Catalog Task level...
var app = new GlideRecord('sysapproval_approver');
app.newRecord(); //newer method than initialize(), but either still works
app.approver = '[sys_id of user]';
app.sysapproval = '[sys_id of sc_task record]';
app.source_table = 'sc_task';
app.document_id = '[sys_id of sc_task record]';
app.state = 'requested';
app.insert();
