Hi @MoonlightBae, welcome to the community!
You can transfer data from your internal memory to your SD-card by going to: Settings > Storage > Transfer data to SD card > Check the boxes for what you want to transfer > Transfer. If it is not possible to check any of the boxes, there is nothing on the internal storage that can be moved to the SD-card.
If you are getting an error message when trying to transfer files I would like to know what that error message says. If you can, take a screenshot of it and post it in this thread. You can take a screenshot by pressing and holding the power key on the side your phone, then press take screenshot.
