ah fixed it , thanks bro

It still needs tweaking in 1.21.1

Check your firewall or any other antivirus softwares that could block downloads. You can also try manually downloading the file from the URL given in the error and then placing it in the path that it says on the error. Also make sure to close your launcher and open it again when making changes

You can follow the "Other launcher" instructions here: https://aristois.wiki/modpacks/
You will need to select the folder that the mods folder is located in (because Aristois will include the /mods/ dir). Usually it's the name of the modpack

Correct. You cannot have fabric installed in the instance

https://paste.ee/p/eXvjQ

You have to download and install the donor client yourself from the website

It's a conflicting mod, but I can't really tell which. Something that modifies NBT data