How do I add checks from previous year that cleared, but not account for them since they are already accounted for in desktop? (started new with QBO)

Thank you for getting back in touch, @bulldogequipment.

You’ll have to manually upload those transactions into QuickBooks Online that cleared in 2020 to be able to reconcile. Once you download the transactions, you’ll need to download, match, and categorize the bank transactions. 

You’ll need to enter a bank account opening balance.

To create the opening balance when you set up the account.

1. Select the Gear Icon and choose Chart of Accounts, then pick New.
2. Under Detail Type, choose the type of account: Cash, current, savings, etc.
3. Type in the appropriate account name.
4. Enter the appropriate information for the opening balance and the date as of.
5. Click Finish.

Then if you didn’t enter an opening balance when you created the bank account, you can enter it as a deposit.

1. Go to the + New button.
2. Hit Bank Deposit.
3. Locate the correct bank account.
4. Backdate the deposit to reflect the opening balance.
5. Leave the Received From blank.
6. In the Account field, select Opening Balance Equity.
7. (Optional) Enter a memo, such as "Opening Balance".
8. Enter the appropriate amount.
9. Pick Save.

For further information about entering a bank account opening balance, follow the link below:

https://quickbooks.intuit.com/learn-support/en-in/help-articles/entering-a-bank-account-opening-balance/00/381389

Lastly, you’ll want to reconcile your account.

If you have additional questions, feel free to comment.