Why can I understand Spanish but not speak it?

Understanding Spanish is recognition: you see or hear a word and your brain matches it. Speaking is recall: you have to produce the right word, ending and order, fast, with someone waiting. Those are different skills, and you only get good at speaking by speaking.

It's very common

This is probably the most common complaint from people who study Spanish with apps, shows or podcasts. They can follow a lot, then freeze when asked a simple question. Nothing is wrong with you. You've trained one skill much more than the other.

Think of it like knowing a song by heart versus being able to sing it. Hearing it a hundred times makes you recognize it. It doesn't mean your voice will do it perfectly.

Why the gap exists

Passive vocabulary is bigger than active vocabulary. You recognize far more words than you can pull up on demand. That's normal in any language.
Speaking needs grammar in real time. When you listen, context helps you. When you talk, you have to pick the right verb form yourself: fui or era? ser or estar? See preterite vs imperfect for one of those choices.
Pressure. Someone is waiting, and your brain is also busy being nervous. There's a whole page on the fear of speaking.
Not enough practice. If you've done hundreds of hours of listening and almost no speaking, the numbers explain it.

How to close the gap

I'd start small, with things that don't need another person.

Shadowing. Repeat what you hear a second behind the speaker, copying rhythm and sounds. It makes the words come out of your mouth. See shadowing. For material, I'm biased, but Langtok (the app I built) works for this: short stories you can slow down and follow along with the highlighted words.
Talk to yourself. Narrate your day: Ahora voy a hacer café (Now I'm going to make coffee). Describe what's around you. It's awkward for a week and then it isn't.
Answer questions out loud. After a story or a show, say a sentence about it. Me gustó el final (I liked the ending).
Write, then say it. Write three sentences about your day and read them aloud. Writing gives you time to get it right, and speaking makes it stick.
Find a real person. A tutor or language exchange partner gives you the pressure and the feedback. See practice speaking by yourself for solo ideas first.

Use what you already have

You probably know more than you think. A trick: don't aim for new words, just use the ones you already recognize. Make short sentences with simple words, like Quiero ir, pero no puedo (I want to go, but I can't). Simple and correct beats fancy and stuck.

Silence is a starting place

If you've spent months listening only, don't feel guilty. A listening-first approach is actually well supported, and many people feel it builds a good foundation. The point isn't to blame it. It's that at some point you need to turn recognition into production.

What to expect

The first few times, you'll be slow and make mistakes. That's fine. In a few weeks of regular speaking, sentences come faster, because the words you've heard so often start to come out. Understanding gave you the material. Now you're training the delivery.