If an error message appears during content download, for example, "Can't connect to iTunes Store.", it is possible that there isn't enough storage on the device.

To check storage, go to the Settings app, tap "General" > "iPhone Storage". If there is limited area of light gray on the right side of the bar, or if the "Used" on th upper right of the bar is close to the total storage, it means there is little storage left. There is also a detailed explanation from Apple.

https://support.apple.com/ja-jp/HT201656

If there is little storage left, tap and enable "Offload Unused Apps" to free up storage automatically. Also, from the list of apps underneath, select the app that hasn't been used frequently, and tap "Delete App" to free up space.

Once you have enough storage, start "Dictionaries by Monokakido", go to "Collection", tap "Store" on top right, display "Dictionary Store", and from "Restore" on top left, please try downloading the content again.