Hi @M4AquaOM, welcome to our forum!
There is a limitation so you can't fill the entire internal storage up completely.
If there would be 0% free space on the internal storage, apps can't save any data at all and you can't receive text messages and so on. That is why we also recommend that you have at least 5-10% free space on the internal storage of your device so cached data from apps and so on can always be saved when needed.
In order for you to install new apps, you will need to free up some storage on the internal storage and a good place to start is by installing updates for the pre-installed applications that you don't use. This can be done in Settings > Apps > name of the app > tap the three dots in the top right corner > Uninstall updates.
Important to remember is to not update these again in the Google Play store.
I hope that helps!
