are there places where one can pratice riding a bike?

I guess an empty or near empty parking lot is way too obvious. 🙄  You can also see if there's any designated bike paths or trails in your area.  Simple as Google maps.  Just click on the little bicycle symbol.  Example from the St. Louis area.  They're highlighted in dark green.  https://goo.gl/maps/h476C5FfsEmNyaUUA  

On a serious note...when you do get ready to move back onto the streets, there's a 3 part series on YouTube I HIGHLY recommend.  It's called Cyclist's Eye View.  I'll supply part 1 below.  Parts 2 & 3 can easily be found.  The absolute WORST thing you can do is think you are NOT part of traffic.  YOU ARE TRAFFIC!  You have the same right to be on the streets as any car, truck, bus or motorcycle.  Bicycles are legally defined VEHICLES.  And in many places, riding on the sidewalk is ILLEGAL - especially in a business district.  Please note that in the videos, the cyclist NEVER hugs the curb or rides close to parked cars.  Riding next to the curb puts you out of the line-of-sight from most (tunnel visioned) motorists.  And riding next to parked cars is an invitation to get "doored".  All it takes is one fool NOT to look before opening a car door.  You...A) run into the car door & get injured.  Or...B) bounce off the car door back into traffic & get RUN OVER by the next passing vehicle.  

You can also take lessons either in person or online from Cycling Savvy.  https://cyclingsavvy.org/   Here's the video...  https://www.youtube.com/watch?v=ZFjCza5e1kw   

We taught our children to ride bikes in the parking lot of a business when it was not open. There were no parked cars, no traffic. We used the aisles as lanes and got them to steer, signal turns, etc. as if they were on a street.

If fear is a little bit of a problem, add knee pads and elbow pads to your safety gear, which should include a helmet. If you fall, you'll be okay.