Check all your connections. I’ve found that the voltage divider method is a little hit and miss.
A coulpe of other things to try.
Try putting a small delay (say, 100ms) after Serial.begin. SD Cards take a little time to ‘warm up’ before they can be written to/read.
I see you’ve got a pin set for the memory cards power. That should be connected to an NPN transistor and the 3.3v output of the arduino then the card.
So, base to pin 8, collector to 3.3v, and emitter connected to the SD cards Vcc.
I also found that simply reformatting the card sometimes sorted problems.
Also, inserting the card while the device is on will generally result in a short, so turn off/unplug the Arduino, put the card in then turn on/plug in.