According to your guide, www.manager.io/guides/fixed-assets, I should be able to set an asset which has been created in the fixed assets section, to the purchase invoice, however there is no drop down to do that.
How did you create Furniture and Fixtures? This should be a control account of “Fixed assets, at cost” (see guide you referred to). Also you need to enable the Fixed Assets tab and create each fixed asset that you will purchase using the purchase invoice.
See also the old guides at https://www2.manager.io/guides/9106
Please show an edit screen of one of your six created assets in the assets list.
Also please show your Fixed Assets control accounts in Settings as you wrongly created an account in the Chart of Accounts. The control accounts will automatically appear in the Chart of Accounts.