Mae is the 'is' in the sentence.
Mae'r comes from combining 'mae' with 'yr'(the).
With two vowels next to each other, one is removed and the phrase is contracted to "mae'r"
It should not accept that.
If you are using the smartphone app, make sure that you are using the latest version just in case there was an earlier version with an error of some sort in it.
mae is the verb here. Have a look at the 'Hints and Tips' notes for the previous section 'Present Tense 3'.