Direct to Garment (DTG) printing is suitable only on products made from cotton (or poly-cotton blends, with a higher cotton percentage of cotton).
DTG printing is not suitable for polyester garments.
This is due to the printing process, where ink is injected into the fabric; as cotton is the only fabric that will allow the ink to remain in the garment.
For polyester products your print work will likely be Digital Transfer, Vinyl or Screen Printed depending on the quantity and the most suitable process.