In many (most?) companies you do not need a degree or certificate; what you need is demonstrated skill. Technical writing is not a super-common degree to begin with; many technical writers have degrees in English or computer science or other fields. I had a writer on my team once whose degree was in history. Further, for very technical documentation, employers are looking for both writing skills and the ability to understand complex technical topics. Having a head start on the complex technical topics can help.
That demonstrated still is important, though -- you have no degree to back you up, so you're going to have to show that you are a capable technical writer. ("Show, don't tell" applies to careers too.) That means you'll need to build a portfolio, and if nobody is currently paying you to do documentation, this probably means doing some side projects or participating in open-source projects. I wrote more about that elsewhere.
Finally, I said "many" ("most?"). Some companies and some sectors are more regimented and expect a degree. In my experience, US government contractors are like this. You will probably have better luck looking at smaller companies and start-ups.