Welcome to the club!
Please have a look at the thread XIAO BLE wont compile ArduinoBLE- Arduino IDE.
There are two packages available for the Xiao nRF52840
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Seeed nRF52 Boards: release
1.1.3does not work; use1.1.1instead. The procedure is For Seeed nRF52 Boards Library | Seeed Studio Wiki. - Seeed nRF52 mbed-enabled Boards is based on Mbed-OS and requires a different Bluetooth library. The procedure is For Seeed nRF52 mbed-enabled Boards Boards | Seeed Studio Wiki.