Sounds to me as if you have connected your loan account for online transaction downloading.
At least in Quicken for Windows, an online-connected loan or mortgage account does NOT have a transaction register. All data shown in the account come from whatever information the bank downloads to you ... if this process works at all.
As a result of being connected, the scheduled payment transaction reminder cannot transfer the amount of principal paid into the (non existent) account register and must use a category, usually something like Loan:Principal, instead. The category seems to vary with the Loan Type you selected when creating the loan account in Quicken.
Effective with Quicken Windows 2018 and newer you can deactivate an online-connected loan account and regain full control over your transaction register. However, you should still review the Scheduled Reminder (or Memorized Payee List entry) associated with the monthly loan payments to ensure they now transfer Principal to the loan account register and not to a Category.
If you're just experimenting to set up a new loan account register in Quicken, it might be best to start over with a new "offline" (manual) loan account.
The transaction register for an offline account will be available in the account's Payment Details tab. While setting up the account, be sure to set up and use the Scheduled Reminder for making your monthly payments from the checking account. It will have the principal and interest amounts calculated for each payment, +/- a few cents difference which you will have to correct every month, based on the loan statement.
If you have a grace period before making the first payment, set up the loan as of the real first payment due date, not as of today's date or when you signed the contract. If there are no hidden surprises on how the bank calculated the loan and you have defined it correctly, there should also be no balloon payment at the end. Use the monthly loan statement to adjust payment amounts as necessary.
If what you have now works, more or less, go with it. Quicken can handle regular mortgages very well, because they follow a fixed set of rules. Car loans are a different situation, however. It all depends on what the bank does, how and when it records a payment transaction and how it calculates interest. The important part is that your recorded payment transactions' principal and interest amounts match the bank's statements, to keep your records in sync with the bank every month
The loan account register only concerns itself with the monthly payments of Principal. Interest is part of the monthly payment (see the Split detail in the scheduled reminder) and is paid directly to the bank. It does not visibly flow thru the loan account itself in the form of separate transactions. You can find how much interest you already paid by looking at the category used to record the interest payment, usually something like Auto Loan:Interest. The Payment Schedule in Quicken will show you how much principal and interest you will pay over the life of the loan.
If the loan is set up correctly, and if you are using the Scheduled Reminder created by the loan setup process to make payments from your checking account, then
Here's an example of how this would look like in the checking account transaction:
Line 1 is principal amount, line 2 is interest, Line 3 is defined as "additional principal payments" and may be blank or 0.00 or, in my example, it was used to show the difference in Quicken-calculated amounts and actual amounts as shown by the loan statement. Have to account for that difference somehow, you know. (And in this example the interest amount is low because this was the last payment prior to the loan being paid off ... in case you're wondering)
