I entered cost of vehicle through Expense tab and associate the expense with a project, but after created the cost, the project doesn't show.

Hi there irene-500plus-ca,

It's vital that your purchases be correctly entered to be represented in a project. QuickBooks Online is a great tool that allows you to add various sales, expenses and hours to a project to calculate the profitability of this job. I can provide some information about expenses and projects.

In order for expense transactions to properly show up in the project, they must affect an expense account. If you've chosen another category to affect in your transaction such as an asset or a liability, that may be why it's not showing up properly in the Projects module.

If you've created the transaction as a Journal entry instead of an expense, that may also be why it isn't showing up on the project itself.

For further assistance with entering the transaction so it shows up in the project properly, I recommend reaching out to the QuickBooks Online support team outside of the Community. They can view the transaction that you've entered to see why it isn't affecting the project. You can reach them using one of these methods.

Schedule a Callback or start a Chat: click (?)Help in the upper right > type and enter "Contact support" into the QB Assistant > click Contact Us > explain your situation > click Let's Talk > choose Get a callback or Start a Chat. 
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If you have any other questions feel free to reach out here.