Uh! Around here, many parents ask
their child for help with modern tech.
BR's question is a good one.
WERE you trying to do a conversion (C) or just send a book to your device (Conversions can be done as needed, on the fly).
Your Profile says Kindle. Including model and Gen helps us be more detailed to fit that device.
It may be that you had Calibre poorly configured. (preferences: Behavior: Preferred Output: AZW3 is common for Kindles )
Notice below that is a list of formats that Calibre can handle. Some provide nicer conversions than others (IMHO leave PDF pretty far down).
The order is what it looks at to choose from, if multiple formats exist for that book.