The default is as follows when creating tax codes (In Account group ) we can select only tax payable & capital account ,How we can map account group to Expenses under Tax field
You cannot. Taxes must always be posted to a liability account, because taxes collected are not your income, but are owed to the tax authority. Taxes paid post to the same liability account to offset taxes collected. They are not your expense.
The foregoing comment assumes the tax is an offsetting type, such as Value Added Tax. If a tax is not offsetting, you should not use a tax code. The tax is simply part of the expense. In that case, you should enter the full amount paid, including tax, and leave the tax code field blank.