There is no definite article present, so the adjective follows the strong declension and has -e in nominative plural (similar to the -e in the article "die").
If there were a definite article present, the adjective would follow the weak declension and would have -en here.
You can see the explanation here:
A better correction would have been to "We are strong women".
"woman" (with an -a-) is the singular, "women" (with an -e-) is the plural.
"we" is several people, so you have to use the plural form "women".
The English word "ladies" is also accepted as a translation of Frauen, though, the preferred translation is "women".
Unfortunately, Duo is not always very good at correcting mistakes to the "closest" appropriate correct translation.