I designed a very simple 4*3 keypad system controlled by an ATMega32U4-MU MCU, using Adafruit's Itsy-bitsy 32U4 as reference (attached is the schematic of my circuit, while Adafruit's circuit can be found at https://learn.adafruit.com/assets/49818). There is a 16 MHz oscillator on board (FA-20H 16.0000MF10Z-AJ0) with 22pF caps.
To program the circuit, I am trying to use an Arduino Uno set as ISP, using the following link as reference: https://www.designedbycave.co.uk/2020/ItsyBitsy-Bootloader/ Connections:
Arduino -> Atmega32u4 board
5V -> 5V
GND -> GND
10 -> RST
11 -> MOSI
12 -> MISO
13 -> SCK
I tried burning bootloader using both Arduino IDE (set board to Itsybitsy 5V 16 Mhz and later on Arduino Leonardo) and WinAVR CLI (Caterina bootloader downloaded from https://learn.adafruit.com/introducting-itsy-bitsy-32u4/downloads). Both show the invalid device signature verbose error.
I was having similar problems with an ATmega328P board earlier, which I solved by connecting pin 10 of Arduino directly to RST pin of target board instead of DTR pin (many people suggested oscillator issues, connection issues, circuit issues, capacitor issues etc. but those were not right). But here, the RST pin is already directly connected to RST pin of the MCU.
I checked for circuit error and shorts but found none. Everything was electrically intact. I made three of these boards and all three have the same issues.
Kindly look into the schematic and suggest what's wrong.