Hello there, @Katerina_.
I'm here to join the thread and help share a little more information about merging accounts in QuickBooks Online (QBO).
Yes, accounts can only be merge if they have the same type. QBO creates some accounts for the company by default and creates other special accounts when certain features are turned on in the settings. You can't modify or delete these accounts.
Since QBO created the Sales Tax account, you can't modify or change the detail type of this account.
For more information about merging accounts, you can check this article: How to merge accounts, customers, and vendors.
In addition, you may also check this article for more information about managing default and special accounts in the chart of accounts: How to manage default and special accounts in the Chart of Accounts.
I'll be always here to help you if you have any other questions about managing accounts in the Chart of Accounts, just add a comment below. Wishing you the best!