When you open your project site https://maksymilianmroz.github.io/react-todo-app/ and do a full refresh with the developer tools of your browser enabled, you will see that all javascript and css references result in a 404. This is because all url all prefixed with maksymilianMroz.
eg https://maksymilianmroz.github.io/maksymilianMroz/react-todo-app/static/css/main.fbf7fb8c.chunk.css
where it should be https://maksymilianmroz.github.io/react-todo-app/static/css/main.fbf7fb8c.chunk.css
This is caused by the homepage property of your package.json file, as shown in the output of npm run build

If you adjust that property in the package.json your build will be fine.
{
"name": "react-todo-app",
"version": "0.1.0",
"private": true,
"homepage": "https://maksymilianMroz.github.io/react-todo-app",
...
}