The “Not Enough Available Capital” warning occurs when a trade is attempted and the bot does not have sufficient funds available to cover the position.
Although there may be enough capital allocated to the bot for any single trade, some of that capital may already be in use for other opened trades. See the Capital section of the bot's Dashboard for a description of currently available funds.
You can always edit your bot's allocation in the Safeguards section of the Settings tab.
More details on position allocation can be found here.