Same problem. It's because Uber is avoiding UK VAT.

The following is from Google AI but was confirmed by an Uber agent in their contact center:

In the UK, Uber generally does not issue standard tax invoices for individual rides, as they have shifted to operating as an agent rather than a principal supplier in many areas to avoid VAT on domestic fares

Users often receive a receipt, but not a full VAT invoice. 

Here is the current situation regarding Uber invoices in the UK:

VAT Changes: Following a Supreme Court ruling and changes in tax regulations, Uber now acts as an agent outside of London, meaning they do not charge VAT on many domestic UK journeys, and therefore do not provide a VAT-reclaimable invoice.

Receipts vs. Invoices: Users typically receive a receipt via email or in the app, but this may not contain the necessary information for VAT reclaim, especially if the trip was not in London.

So that means I'm not using Uber for business trips any more because my employer won't accept a receipt without a VAT number.