What's happening is the gravity of the planet (rings) is forcing you out of supercruise.
I read this message as "dropping" (too close!) - too close to the rings, as you approach the resource site. First, fly direct away from the rings (vertically up, and away from the planet). Hopefully this will get you into supercruise. Once you're away and cruising, re-approach the site from above (or below), keeping your distance from the planet, and from the rings until the last minute. You're looking for the blue "safe disengage" alert which locks the target and will drop you in the right place.
(SC is a warp drive and engines are less able to create a warp near large objects. This "mass locks" you, near stations and large ships so you can't even get started and will slow you down as you fly near planets (that red "slow down" message is not an instruction; warp engines are fighting against a gravity source and in the end you get an overspeed). In dropping, too close, your drive fails and the collapsing warp field can (if you're going too fast) cause hull damage. Take your time on approach.)
Good luck.