Translation:That party was not much fun.
TL;DR Don't use ないでした.
I agree with dexpanyol. Indeed, ないでした is strange to most Japanese. Although, I did find an obscure reference that it would not be considered strange to those from Kagoshima prefecture. And, due to the influence of the Meiji restoration and Satsuma (Kagoshima), it can be found in Meiji and Taishō era literature.
Amari means slightly with a negative connotation, can't think of a fitting english word. And Tanoshii means enjoyable, fun, something like that, kuarimasendeshita is a formal way to turn it into its opposite. deshita is what you put after verbs to mark that this event happened in the past.
Hi, 楽しい is an i-adjective (adjective that ends in い) To make an i-adjective negative (e.g. Not fun) you change the い to く + ない in plain form. However, this is too informal. In formal setting, ない is replaced with ありません so it goes from 楽しくない to 楽しくありません
This sentence is interesting because it is past tense. In the most plain form, it could be written 楽しくなかった (past tense of ない). However, past tense of ありません is ありませんでした
That is how you get the final 楽しくありませんでした (fun-not-past) => "was not fun (polite/formal)"
There is a bug. When I entered :"That party was not fun" I got corrected to write "That party was not interesting ". Now I wrote "That party was not interesting" and I was marked wrong with the right answer being:"That party was not fun".... I screenshot that as I wanted to be sure I am not getting crazy (×_×)...
From my japanese class they taught me that "tanoshii" is an adjective that is a "-i" adjective and thus, it is modified into negation and past tense by saying: tanoshii desu / tanoshikatta desu / tanoshikunai desu / tanoshikunakatta desu. It is confusing that in this sentence they are using a construction with "arimasen deshita", that is reserved for "-na" adjectives, that is what I have been taught.