Error message "A tax mapping for [tax] is required"

The message means that you have to set a mapping for the tax in Bookeo to the relevant tax in Xero. 
To set the tax mapping:

1. Go to Settings>Integrations

2. Click on Xero

3. Tick the Enable Xero integration checkbox

4. In the Tax Rates section, map taxes in Bookeo to a tax rate in Xero.

In Bookeo, taxes are set up in Settings>Taxes.

In Xero, Tax Rates are set in Settings>General Settings>Tax Rates

5. Click on the New button [a pop up window will open]

6.  In the Bookeo section, select the Tax(es) set up in Bookeo, in the Xero section, select to which Tax Rate they correspond in Xero.

7. Save

How to map 2 Sales Taxes in Bookeo
If you have set up 2 Sales Taxes in Bookeo, for example, a Federal and a State Tax, the 2 taxes will correspond to one single Tax Rate with 2 components in Xero.

If for some services or customers you apply only 1 tax in Bookeo - for example only the Federal or the State tax -, you will have to create a new mapping in Bookeo, for the 1 tax to the 1 tax in Xero (not the 2 together)

Map sales without Sales Taxes
You will also have to set a mapping for sales where no taxes is applied. In this case, do NOT tick any tax in the Bookeo section, and select the relevant Tax Rate in Xero