One of the requirements in order to request an ISA transfer from another bank/broker is to have an opened ISA with us.
If you are transferring a current year subscription from an active S&S ISA you should wait until the full completion of the transfer process before funding your newly-created Trading 212 ISA.
On the other hand, if you have requested a previous-year transfer only and you already have an active S&S ISA elsewhere*, no deposits should be made towards your Trading 212 ISA account. You can contribute only to one S&S ISA during the same tax year.
*An ISA is activated whenever an amount is deposited to it during a certain tax year.