To start Portfolios with a date earlier than today you need to use the "Apply cash balances" option. The "No cash balance" option only allows you to enter transactions for your current holdings. When you create a Portfolio, you need to enter a start date for it. This should be the date of the earliest transaction that you wish to record, not the date you're starting to track your transactions using the tool. This is necessary for calculating your money-weighted returns correctly.