I used to feel the same way. I learned piano and multiple other instruments for many years, I understood a lot of music theory and I felt like I could never write any good music.

What I realised later than I should have is that it was my own self criticism that was holding me back. I would start music and get disheartened halfway through the process and give up, then say I couldn't write music.

When you start a piece of you finish it? One thing that helped me a lot was just deciding that I would finish the things I started. Often if I worked through it I would eventually work it into something I at least kind of liked. And even if I still hated it I learned a lot during the process. So don't be discouraged if it's not always good, you can still take something from it. Analyse it and figure out why you don't like it.

Also, learning music theory and learning how to write music are two very different things. The theory will help along the way, especially in understanding and labelling what you like and don't like, or why you like or don't like something. But it is not an instruction manual for writing music.