The car is not going into gear, or the gearing slips when switching from Reverse to Drive mode.
Start the vehicle for 10 minutes, then check your transmission fluid level, as per your owner’s manual. If the fluid level is low, then you need to add fluid, and have your system checked for leaks. Aside from this possibility, you should have your vehicle scanned for Diagnostic Trouble Codes; you may need a new transmission. An inspection of your shifting difficulty issue from a qualified technician, such as one from YourMechanic, can help you determine the cause, and fix your issue.