Do you mean after 20 mph, it seems like the car goes into neutral and the engine revs? Or, does it feel like it won't shift to the next gear?
Either way, the first thing I would check is to make sure there isn't a leak and make sure the fluid is full.
What to do is allow the engine to reach operating temp. Park on a flat surface, place the parking brake on, and pull the dipstick for the transmission and check level. The dipstick will be near the firewall and on the passenger side under the hood.
When it is checked, the engine should be running and at an idle. I believe this one needs checked in neutral, but it will say that on the dipstick.
Let me know what you find or if you have other questions.
Saturday, May 30th, 2020 AT 7:14 PM