Invoice without a company

Xolo doesn’t work.
Invoicing through a platform like Upwork is a solution if you actually find your clients through that platform.
Otherwise, those public invoicing factories are dangerous, as the companies they use become quickly blacklisted.

I talked time ago about xolo and other similar services (use the "search" function).

Moreover, publicly offering to invoice third party clients and collect payment on their behalf is a regulated activity in many countries.
For example, Spain bans unauthorized worker leasing under Article 43 ET. Germany requires an employee-leasing permit under AÜG §1, and §9 voids contracts without it. Italy punishes unauthorized intermediation under Legislative Decree 276/2003, Article 18. France allows it only through regulated portage salarial companies under Labour Code Article L1254-24.
So, in a lot of places these invoice factories are plain illegal by design.

Not to mention the obvious money laundering problem: you are inserting a paid intermediary between the client and the real service provider for no commercial reason except disguise and convenience. That creates third party payments, false invoice optics, beneficial owner confusion, and possible money transmission issues.

In short, you risk serious troubles.
Better to pay a bit more and use a proper structure.