My Cardano account shows that I have 100 ADA available but, when I try to send this amount, it only lets me send 80 ADA.
This is most likely because you have non-ADA tokens in your Cardano account; all tokens (including NFTs) are assigned an ADA value known as ‘min-ada-value’. This ‘extra’ ADA is added to your available balance but can’t be used in a transaction.
Instead, this small amount of ADA is attached to the token when it’s transferred, meaning your available ADA balance decreases slightly when you transfer out any tokens, and increases slightly when you receive them.
To determine how much ADA you have available to send, click MAX in the Send box, under ‘Amount’. This will show you how much of the ADA in your available balance is usable.