There are a number of reasons why you may not be able to withdraw:
When you sell shares, you won’t be able to withdraw the associated cash until the trade has settled. Settlement is typically completed 2-3 working days after the trade has executed.
If you have only ever added cash to your account using Apple or Google Pay, you’ll need to verify your linked bank account by making a bank transfer (minimum £1) before you can withdraw to that account.
If you have an ISA with any outstanding subscription payments, you will need to pay for any unpaid invoices before you can withdraw.