Here the rose is a noun and type is a different noun, not an adjective. Each noun has its own gender in Spanish. Type is masculine and rose is feminine, so it will always be "el tipo" and "la rosa". This is why you say "un tipo de rosa", "un tipo de camisa", etc.
Why not "una tipa"? Logic being "rosa" as a flower is a feminine noun when it means a type of flower and when it means the color pink it is a masculine noun. Here it is a flower. "un tipo de rosa" has 47,000 results google results and "una tipa de rosa" has a mere 29 so I do assume duolingo is right. Ah never mind I see "tipo" is masculine when it means "type".
Only flor means flower. Flora means the same thing in Spanish as it does English, plant life.
I wish Duolingo would give more sentences like this. Easy and I feel proud when I've got it right.