These are the two commons reasons.
1. Your Trade Hasn't Settled
To withdraw money from a 'sell' trade, the trade must be settled. Settlement is referred to as T+2 settlement and is a standard process governed by the ASX. Funds will then be released the following business day.
An example: you sell your shares on a Tuesday. The trade will have settled by Thursday. You can then withdraw your funds on Friday. This is assuming there are no national public holidays.
2. You've Only Just Deposited Money
Money you have deposited into your SelfWealth account will be reserved in your account for 72 hours from the time of the deposit. This means you will need to wait 3 days before you can withdraw the funds back to your nominated bank account. You can use these funds to trade or transfer to USD if you wish.
Once a transfer or withdrawal has been submitted it cannot be cancelled or adjusted.