The BOOTSEL button on the Pico is not connected to a standard GPIO, so it cannot be read using the usual digitalRead function. It can, however, be read using a special (relatively slow) method.

The BOOTSEL object implements a simple way of reading the BOOTSEL button. Simply use the object BOOTSEL as a boolean (as a conditional in an if or while, or assigning to a bool):

// Print "BEEP" if the BOOTSEL button is pressed
if (BOOTSEL) {
    // Wait until BOOTSEL is released
    while (BOOTSEL) {