Hello Villery,
If dictation isn't working, the first thing we suggest checking is that you don't any Content & Privacy Restrictions enabled:
Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support
If that doesn't help, let's remove any keyboards you may have added and re-add them. You can do so by tapping Settings > General > Keyboard > Keyboards. Tap "Edit" in the upper right corner to remove. Then, tap "Add New Keyboard" to add them back to the list. Once you've done this, restart your iPhone and see if you're able to enable dictation again.
If the above doesn't work, contact Apple Support for further troubleshooting.
Take care.