You can only transfer funds from Flexible to Fixed Account, not the other way around.
What can I do if I want to transfer assets from a Flexible Account to a Fixed one?
Choose your Flexible Account balance as the funding source when creating a Fixed Account. You can also proceed to Instant Transfer directly from the Flexible tab and the withdrawal form for Flexible Account.
Refer to our article about Instant Transfer for more details.
What can I do if I want to transfer assets from a Fixed Account to a Flexible one?
You should wait till your Fixed Account holding period ends. When the funds are credited to your Wallet, create a Flexible Account with the desired conditions.
Please read our article about creating a Flexible Account for more information.