The trial is 7 days of free access. It is issued once per account, from a button in the @obfsnbot bot or in your account, in the subscription block on the main page.
The same servers and locations as a paid subscription, both protocols (VLESS Reality and Hysteria2) and one link per account. There is no separate cut-down server set for the trial. The personal Telegram proxy comes only with a paid subscription and is not available on the trial.
Most likely the trial has already been used on this account, and it is not issued twice. Your account and the bot are the same account, so there is no way to take a trial separately "in the bot" and "on the site". The button is also hidden while an active subscription runs.
If the trial is used up, free days are still available other ways: invite friends with your referral link, complete tasks and claim the rewards, or subscribe to the @obfsn channel. Or take out a subscription on the payment page.
Pay for any term on the payment page. The subscription becomes paid and your unused trial days are added to the paid term, so there is no need to pay on the last day of the trial.
The subscription link does not change: the app simply keeps working, and refreshing the profile in it is enough. If the trial ran out and the app stopped connecting, refresh the subscription in the client after paying and the working servers come back.