It's in the header file, esp_now.h (hover over the file name in the #include statement, right-click, and choose Go to Definition)
#define ESP_ERR_ESPNOW_BASE (ESP_ERR_WIFI_BASE + 100) /*!< ESPNOW error number base. */
#define ESP_ERR_ESPNOW_NOT_INIT (ESP_ERR_ESPNOW_BASE + 1) /*!< ESPNOW is not initialized. */
#define ESP_ERR_ESPNOW_ARG (ESP_ERR_ESPNOW_BASE + 2) /*!< Invalid argument */
#define ESP_ERR_ESPNOW_NO_MEM (ESP_ERR_ESPNOW_BASE + 3) /*!< Out of memory */
#define ESP_ERR_ESPNOW_FULL (ESP_ERR_ESPNOW_BASE + 4) /*!< ESPNOW peer list is full */
#define ESP_ERR_ESPNOW_NOT_FOUND (ESP_ERR_ESPNOW_BASE + 5) /*!< ESPNOW peer is not found */
#define ESP_ERR_ESPNOW_INTERNAL (ESP_ERR_ESPNOW_BASE + 6) /*!< Internal error */
#define ESP_ERR_ESPNOW_EXIST (ESP_ERR_ESPNOW_BASE + 7) /*!< ESPNOW peer has existed */
#define ESP_ERR_ESPNOW_IF (ESP_ERR_ESPNOW_BASE + 8) /*!< Interface error */
You can find the value of ESP_ERR_WIFI_BASE the same way. Then:
Hint 1: 100 in hex is 0x64.
Hint 2: no esp_now_add_peer calls in your sketch.