designator order for field 'xxx' does not match declaration order in 'yyy'

This error occurs when the "order of declared members" is not in the right order.
I don't know the rules of this language enough, but it would appear there is a fixed order when you do an inline declaration:

This is correct, for example:

camera_config_t config =  {
    .pin_pwdn = PWDN_GPIO_NUM,
    .pin_reset = RESET_GPIO_NUM,
    .pin_xclk = XCLK_GPIO_NUM,
....

While this one (reset and pwdn are reversed) generates the error:

camera_config_t config =  {
    .pin_reset = RESET_GPIO_NUM,
    .pin_pwdn = PWDN_GPIO_NUM,
    .pin_xclk = XCLK_GPIO_NUM,
    ....