@yellowelise

You have to do / when referencing the root in Arduino SD libraries..

I've modified your example to output to the screen instead of serial and fixed it for you.. (I left the syntactically redundant exists, try create anyways, exists code in as that's what the original example did, it probably needs another if)

#include <M5EPD.h>

M5EPD_Canvas canvas(&M5.EPD);

File myFile;
void setup(void)
{
  M5.begin();
  M5.EPD.SetRotation(90);
  M5.EPD.Clear(true);
  M5.RTC.begin();
  canvas.createCanvas(540, 400);
  canvas.setTextSize(3);


  bool ret = SD.begin(4, SPI, 20000000);
  if(ret == false)
    canvas.drawString("SD ERROR...", 45, 100);
  else  
  {
    canvas.drawString("SD OK...", 45, 100);
    if (SD.exists("/screen.txt"))
      canvas.drawString("Screen.txt Exists...", 45, 150);
    else
      canvas.drawString("Screen.txt Not Exists...", 45, 150);
    canvas.drawString("Try create Screen.txt", 45, 200);
    myFile = SD.open("/screen.txt", FILE_WRITE);
    myFile.println("Some text...");
    myFile.close();
    if (SD.exists("/screen.txt"))
      canvas.drawString("Screen.txt Exists...", 45, 250);
    else
      canvas.drawString("Screen.txt Still Not Exists...", 45, 250);
  }

  canvas.pushCanvas(0,0,UPDATE_MODE_DU4);
}

void loop(void)
{

}

It's confusing to tell if its fully charged using the example code as the battery icon is just calculated by looking at voltage.. so it'll jump to 100% quickly after the USB-C is connected.

It will not however be full though. (the voltage is increased by the USB not by the battery being charged)

Leave it charging for a few hours.

I#d love to see this in international versions with a compliant socket.. (the one shown here is not acceptable in the UK)

To go full circle and solve the issue... Are you using platformio rather than arduino?

You need to add:

# Name,   Type, SubType, Offset,  Size, Flags
nvs,      data, nvs,     0x9000,  0x5000,
otadata,  data, ota,     0xe000,  0x2000,
app0,     app,  ota_0,   0x10000, 0x640000,
app1,     app,  ota_1,   0x650000,0x640000,
spiffs,   data, spiffs,  0xc90000,0x370000,

in a new file "default_16MB.csv" in the same folder as platformio.ini, and modify platformio.ini to add:

board_build.partitions = default_16MB.csv
build_flags = 
	-DCORE_DEBUG_LEVEL=4
	-DBOARD_HAS_PSRAM
	-mfix-esp32-psram-cache-issue

Which fixes the RAM issue and allows using the full size canvas.

The problem including in some of the built in examples (HelloWorld.ino) seems to be related to the canvas size.. in the Hello World example it creates a 540x960 canvas (which is the resolution of the display)

If you drop that down to 540x400.. it works fine.. Is there not enough RAM to create a canvas the size of the display maybe?

Did you make any progress on this?

The "HelloWorld.ino" example from M5EPD which I believe is very similar to the one above.. does not work either..

#include <M5EPD.h>

M5EPD_Canvas canvas(&M5.EPD);

void setup()
{
    M5.begin();
    M5.EPD.SetRotation(90);
    M5.EPD.Clear(true);
    M5.RTC.begin();
    canvas.createCanvas(540, 960);
    canvas.setTextSize(3);
    canvas.drawString("Hello World", 45, 350);
    canvas.pushCanvas(0,0,UPDATE_MODE_DU4);
}

void loop()
{

}

Produces a blank page when rendered.