On the first attempt the error message shows:
"This library only supports boards with an AVR, SAM, SAMD, NRF52 or STM32F4 processor."
Cause: The NANO 33 BLE has "#define ARDUINO_ARCH_NRF52840", but Servo.h knows only "#define ARDUINO_ARCH_NRF52"
Correction in Servo.h: Supplement (duplicate):
#elif defined(ARDUINO_ARCH_NRF52)
#include "nrf52/ServoTimers.h"
#elif defined(ARDUINO_ARCH_NRF52840)
** #include "nrf52/ServoTimers.h"**
Result: Servo is accepted, but there are more error messages:
D:\Arduino\libraries\Servo\src\nrf52\Servo.cpp:92:35: error: 'PinDescription {aka struct _PinDescription}' has no member named 'ulPWMChannel'
instance=(g_APinDescription[pin].ulPWMChannel & 0xF0)/16;
...
D:\Arduino\libraries\Servo\src\nrf52\Servo.cpp:123:35: error: 'PinDescription {aka struct _PinDescription}' has no member named 'ulPWMChannel'
instance=(g_APinDescription[pin].ulPWMChannel & 0xF0)/16;
The pin description in g_APinDescription or PinDescription itself may be wrong? I find some different declarations in "variant.cpp" for the different processor families.