Yes the self service checkouts do cost jobs and people will not have as many jobs available and will lose their jobs as a cashier when self service checkouts are installed in the stores.
The self service checkouts do have at least one person overseeing the self service checkouts to prevent stealing and to check ID etc so that means that at least one job is still available to oversee the self service checkouts.
But those self service checkouts mean that that many people will not have jobs available to them to earn a living but those people could find an even better job somewhere else possibly.
Being a cashier is a good first time job but really if you can you should work on getting a better job that pays more since being a cashier is a very low paying job.
Also some stores such as Walmart who have self service checkouts do still have a few other cashiers available for people who want to go to a regular cashier.