Hi there, Rosie Burgon.
Yes, you can only add tags to transactions in QuickBooks Self-Employed.
After you categorize your downloaded or manually entered transactions, you'll have the option to add tags to them.
You may want to create rules for your transactions to categorize recent and past transactions quickly. To find out more about this, check this article: Create rules to speed up reviews in QBSE.
As always, you can post anytime here if you need help navigating your account. The Community is always ready to help.