@Lilygo Disregard. I successfully upload and run 100%. The cause was USB connection, i.e. connecting to USB 2.0 port makes unreliable.

@papbo Does you code have serial port initilization code?

// Set console baud rate
SerialMon.begin(115200);
delay(10);

// Set GSM module baud rate
SerialAT.begin(UART_BAUD, SERIAL_8N1, MODEM_RX, MODEM_TX);

@papbo AllFunctions.ino has following code.

//Initialize SDCard
SPI.begin(SD_SCLK, SD_MISO, SD_MOSI, SD_CS);
if (!SD.begin(SD_CS)) {
Serial.println("SDCard MOUNT FAIL");
} else {
uint32_t cardSize = SD.cardSize() / (1024 * 1024);
String str = "SDCard Size: " + String(cardSize) + "MB";
Serial.println(str);
}

@papbo How do you attache the SD? Does its contact side face to the board? Its connector is not fail safe.

@Lilygo Could you answer? If this is a defect of my device, I want to ask you to replace.

See https://community.lilygo.cc/topic/30/can-t-download-firmware-on-sim7600g-most-of-the-time for detail.

@papbo Can you download firmware every time? It fails 95% on my environment.
Ubuntu-18.04.6
arduino 1.8.13

The failure rate is 95%.I bought http://www.caztech.com/lilygo/sim7600/20211220/DSC_0014.JPG and http://www.caztech.com/lilygo/sim7600/20211220/DSC_0015.JPG at Lilygo official shop. I successfully modify, build, download and run Arduino_TinyGSM/AllFunctions/AllFunctions.ino a couple of times which is confirmed by http://www.caztech.com/lilygo/sim7600/20211220/serial-20211220.png where it tells 180 seconds instead of default 30 sec. When I click upload on Arduino, Arduino can't connect /dev/ttyACM0 as http://www.caztech.com/lilygo/sim7600/20211220/arduino-20212220.png. There is tendency of 100% failure on cold morning and some success on warm evening. Is there anyway to download at 100% success rate?

@lilygo Thank you. I successfully build, download and run all example sketches.

I successfully read SD card on my sim7600g, I mean I have "SDCard Size: 3789MB" when I run examples/Arduino_TinyGSM/AllFunctions/AllFunctions.ino. The SD card is formatted as FAT32 on Windows 10, no partition. I suggest to run one of examples such as AllFunctions.ino first.