If the purchase of a motor vehicle is debited in the machinery account which error is this

A motor vehicle is an asset, and its purchase is a capital expenditure. Debiting an asset account is the correct accounting treatment for an increase in assets.

The mistake is that the purchase was debited to the machinery account instead of the motor vehicle account. Both are asset accounts.

An error of commission occurs when a transaction is recorded incorrectly, such as posting to the wrong account of the same class, or posting the wrong amount. In this case, the correct type of account (an asset account) was debited, but the wrong specific asset account was used. This error would not affect the trial balance because the debit was still made to an asset account.

This is an error of commission.