Talk to them all; the STL, ETL-GE, GSTL, and HR. Constant badgering is about the only way things get done. I was told I was going to be trained for backroom for over a year at my former store. But the ETL-Log kept putting it off until he transferred out of store, and I was never was moved. And it took almost 1 1/2 of badgering before I was even put on the Salesfloor.
If you are a cashier or work the Front areas, you really don't have to be scheduled unless the front needs the coverage. Years ago, I asked my original ETL-GE that I wanted to work Guest Service. And she told me to on come on a free evening of mine so I could learn closing. And then on days where it wasn't a rush and the front had the coverage, the GSTLs let me shadow at Guest Service.
Honestly the hardest thing to do behind Guest Service is dealing with the Higher than Mighty guests, and when to vibe them or when to say sorry but I can't return it. The POS is setup to be pretty straight forward. Just read the training guide.