Nails then tan.
The hand sanitiser used at the start of the nail service would affect the tan, also you'd be touching and holding the clients hands and fingers so could get tan on yourself.
Just ensure the nails are fully dry (if polishing) of have a little cuticle oil on them if enhancements, be careful not to get cuticle oil on the surounding skin though as it needs to protect the enhancement from staining but allow the fingers to tan correctly.