Correct, you can only pay into one Stocks and Shares ISA per tax year. If you’ve already done so with another broker you cannot open another with Freetrade until 6th April 2019 (start of the new tax year) - but ISAs are transferrable between managers, meaning that if you were willing to effectively close the ISA you’ve already opened, you can move it to another broker and it becomes your one for that tax year. This is complicated enough for a blog post, I feel.