It is not possible to withdraw funds which are currently invested. Only uninvested cash can be withdrawn. Withdrawals from the Czech platform are possible only in CZK. You are required to convert EUR to CZK. The tool for this is in the upper right corner (Převod měn) of your platform and DEGIRO will waive the conversion fee. You can convert only the whole amount (no decimals) and manual conversion is not possible at night.
Your balance is the sum of the positions in your portfolio, your cash in the FundShare EUR Cash Fund, and cash in held other currencies. To withdraw your full balance, you may need to sell your positions and then convert funds to your base currency.