Hi Noel,
Sorry for any inconvenience caused.
To purchase your current trial account, here are the steps for your reference:
1. Login to Office 365 Admin Center (http://portal.microsoftonline.com/).
2. Go to the subscription page.
Click Manage and purchase subscriptions under licenses.
3. On the licenses page, click subscriptions.
4. Click the subscription name(Office 365 Small Business Premium Trial).
5. Click Buy now to finish the subscription according to the instruction.
If there are no Buy now button, please also try the following steps to purchase it:
1. On the licenses page, click purchase services.
2. On the Office 365 plans list, you will find the desired service plan, then click Add button to finish the purchase.
If you’ve tried the steps above, and there are any problems, please feel free to post back providing related screenshots for further assistance.
Thanks,
Vivian Wang