If you have a Photoroom design that has a background in place and you want to remove that background, use the Erase feature. It is available as a setting on the background layer and it removes all content from that layer. Visually, it has the same effect as applying a transparent background.
When the background is erased, your design is left with an empty background layer. You can select the layer and use the options to add a new background or you can leave the design with a transparent background.
For detailed instructions on how to erase the background layer, see:
If you're looking for information on how to import a photo and get Photoroom to remove the background, see Remove a background.