I’ve had interviews where I’ve forgotten how to do division.
That’s what 40+ years of programming experience gets you. The panic never goes away 
Interviews are like pants. Sometimes you shit in them. So don’t worry about it.
Hey man, don’t worry. You put yourself out there and took a chance. That deserves a huge pat on the back. Like Dory said in Finding Nemo, “Just keep swimmin.”
But what was the task?
It could be relevant; if you have to build or maintain a custom CMS, finding your way around WordPress in a live coding session can show how you investigate systems and how well you know what a CMS is in abstract terms.
I remember once I bombed an easy interview Python question so very, very bad. I whispered, “I’m going to be reliving this in my nightmares,” and the other guy heard it and laughed.
We’ve all been there mate. Rite of passage.
Coding interviews/tasks can often be complete nonsense.
There can be so many variables it just isn’t a great way to determine a good candidate.
I bombed at one interview for a full-stack React/Mongo position where the ONLY technical questions and tasks were around encryption and math problems, and I was not prepped that those would be the subject.
Move on and believe in what you can do.
Look at the bright side: you dodged a bullet; WordPress isn’t nice to work with.
Just keep applying.
Also, go make a WordPress site and mess with it. Young people will rip on WordPress, but a tremendous portion of the web uses it, and the vast majority of small businesses are running WordPress sites. Most marketing firms and design agencies use it as their main CMS.
Even though it’s meant to be a blogging software, it’s basically the go-to CMS for anything that’s not a complex application for most companies.
Let me assure you, as someone who has been programming for 32 years professionally, and longer as a hobby, when I was recently laid off, I bombed my first few coding tests. One of the tools they have you using are often subpar, think HackerRank and LeetCode; two, often the problems they assign bear no resemblance to the work you’ll be doing; and three, they ask the most esoteric of problems that 99.9% of programmers are going to need to Google.
Keep your chin up; each one you bomb is practice for the next one.
Places that require coding interviews are not places you want to work. My suggestion is to find local programming groups, get to know people, and let them know you. You will find a decent job quickly. Getting a position is more about personal relationships than knowledge or skill. Knowledge and skill are important in keeping the job. Places that have that reversed are hiring the wrong people.
I agree with everyone that filtering out some results may prevent this, BUT be careful about over-filtering. For a lot of the technical interviews I’ve done, I was given a choice on the language/stack. So just because a job description states they use WordPress, or Python, or whatever, you should still apply if it interests you because they may be understanding and accommodating of your unfamiliarity with their preferred tech. Worst case is you fail again like this time, but the interview experience will be worth it.
@Alex
Yeah, the job description listed a bunch of languages – SQL, AWS, Python, etc. – so I thought it would be more general dev work. I didn’t expect it to be mostly WordPress-focused.