Translation:The test yesterday was not easy at all.
やさしくないです。 This would be the negated present tense.
やさしくなかったです。 This would be the negated past tense. Not impolite because of the です.
やさしくありません。 This is how you would find it in books, but I doubt any Japanese person talks like that among themselves. It would make you sound awkward in daily conversations.
'Yesterday' can fit in more than one position in the translated sentence. Better make it right in all of the possible positions, then you won't have to keep coming back to see why this sentence has been flagged.
Great, now I have a grammatical structure to address the Emperor of Japan and say my test was not easy. 10/10
The conjugation is wrong because ない itself follow the conjugation rule of i-adjectives. You can instead use another form "やさしくなかったです". Actually this form was grammatically incorrect decades ago but is now much more common than -くありませんでした. The latter is used more in formal conversations.