Because mòr does not go with a’ Phòlainn here but with the verb tha – it is a predicate of the verb, not an attribute of the noun, and only attributive adjectives change in Gaelic.
- Tha a’ Phòlainn Mhòr anns a’ Phòlainn Greater Poland [a Polish region] is in Poland);
- but Tha a’ Phòlainn mòr Poland is big;
- tha a’ phàirc brèagha the park is lovely;
- but tha a’ phàirc bhrèagha mòr the lovely park is big.
If you look at the top of this discussion you’ll see the apostrophe is there (ie. it’s a’ Phòlainn for Poland, with an apostrophe in the definite article).
It’s just that Duolingo software treats final apostrophe as a punctuation and not part of the word and removes it from the word tiles in the choose tiles in right order exercises. A bit of English-centric assumptions in Duolingo programming, I guess.