Ash here from the Shopify team. Welcome to the Shopify Community!
When your Shopify account is in the trial period, your online store will be password protected. This password can only be removed once you select a plan for your account. We have some more information you can check out here regarding the password protection for online stores. With that being said, you should be able to customize and preview your theme while you're still in the trial period. If you're being prevented from customizing and previewing your theme while on the trial period, could you please show me a screenshot of the step you're stuck on?