I can’t run the code

Textastic offers syntax highlighting for many languages, but it doesn't include compilers or interpreters.

The web preview of Textastic supports HTML, CSS, JavaScript and Markdown files.

For example, if you try to preview a PHP file locally, only the html part will be interpreted by the web view. The PHP part will be ignored. In order to preview PHP in Textastic, you can upload the file to a test server, open the preview screen, switch from "Local" to "Remote" and enter the remote url of your test server.