You can create journal entry directly to vendors or customers in QuickBooks Online. Journal entries are the last resort for entering transactions. However, I suggest entering customer and supplier transactions.
If you still wish to use a journal entry, make sure that you have access to create a journal entry. You may check this reference to learn more about the different options for user roles and access permissions: User roles and access rights in QuickBooks Online.
When creating a journal entry, I also recommend consulting an accountant for the appropriate accounts to use.