Why can't I put a percentage against my car expenses ?

Thanks for posting in the Community space, dwpratt-hotmail-.

In QuickBooks Self-Employed (QBSE), to split a transaction by a percentage, you'll need to sign in via a web browser. 

Here's what you'll need to do:

1. In the left menu, click Transactions.
2. Locate the transaction you want to split.
3. Choose Split from the Type column.
4. Pick Percentage in the split transaction window.
5. For each split, select Business or Personal from the Type ▼ dropdown and set Category.
6. Enter the percentage for each split.
7. To add additional splits, select Add line.
8. Once done, hit Save.

For more information, please check out this article: Split transactions in QuickBooks Self-Employed.

I'm adding this resource that talks about categorising transactions to serve as your guide: Categorise transactions in QuickBooks Self-Employed.

You can always visit us here if you have any other concerns. I'm here to help. Have an awesome day!