Why can't I spend all of my spending balance?

Not all of your spending balance is spendable because part of it is reserved as a mechanism to secure Lightning connections. This mechanism is common among the network and is not specific to Bitkit.

Each connection opened by Blocktank, Bitkit's integrated Lightning Service Provider, set the reserve balance to the minimum of 546 sats. Channels opened by external nodes can have a different reserve balance, but it's usually set to a maximum of 1% of the total connection capacity. The sum of the reserve balance of all of your connections is the unspendable balance.

Remember, the reserve balance will return to your savings balance once you transfer it back or your connection gets closed.