If I still have transaction I didn’t get completed from last year can I still enter them?

Hello, Jennifer100574. 

Yes you can. You can enter it as a Journal entry and make sure to match the payment information posted in QuickBooks. However, to ensure you use the correct account, I'd recommend seeking advice from a professional accountant. This way, your QuickBooks data is accurate at all times.  
 

1. Click the + New, then select Journal entry. 
2. Enter the date of the journal entry. This is when the payment in QBO reflected.
3. On the first line, select an account from the Account field. Depending on if you need to debit or credit the account, enter the amount in the correct column.
4. On the next line, select the other account you're moving money to or from. Depending on if you entered a debit or credit on the first line, enter the same amount in the opposite column.
5. Check the amounts - you should have the same amount in the Credit column on one line and the Debit column on the other. This means the accounts are in balance.
6. Add a note in the Description field so you know what the journal entry is for.
7. Click Save. 

For more information on how to record journal entries, click this article: Create a journal entry in QuickBooks Online.

Then you can match them to those existing transactions in QuickBooks. Here's an article as your guide:  Add and match transactions. 

For future reference, read through this article to learn about detailed steps in reconciling your transactions in QuickBooks Online. 

Please let me know if you have any questions about entering previous transactions in QuickBooks, I’ll be more than happy to answer them for you. Thanks for dropping by, and take care!