Hi, check the JS errors on that page. Press F12 to open console / depurator.
The code to delete user account is a direct link, no matters if permalinks is enabled or not. But, exists a javascript function firstly to open a confirmation window (but if exists JS errors on page, can be possible mess up with that, or not).
You can try use the link without use the ID to that confirmation window, just to test if will delete the user account or not.
Add this line code on user-profile.php file
<a href="<?php echo osc_base_url(true).'?page=user&action=delete&id='.osc_user_id().'&secret='.$user['s_secret']; ?>"> Test delete user </a>
You can comment the javascript function too, just in case, to not interfer with that.
If this direct link not working to delete user account, in that case, some plugin not permit the deletion for some special reason. Maybe user have some premium ads actives (paid with payments plugin or other payments system) and premium time not expired yet?
Create a fake user, login by user and try delete the user account on profile settings to see if will delete or not. Disable your payment plugin too, for test it.
Come back here again to reporte the situation.
Regards