When trying to install a modpack, I received the following error:

2021/05/23 10:09:14 error installing mod file: failed to find slug for project 476520: no project found 476520

This is likely because this mod is not yet in the database of all available mods.
However, this should not be a fatal error, as (in this process) the slug is only used to name the mod when it fails to download.
The slug is not used at all to enable the download, instead the project id and file id are used.
At most this should just produce a line saying it can't find the slug, and continue. And then in the case of a fatal error where the download can't be found, it can describe it using the project id.
Or alternatively, for it to only actually try to find the slug if it fails to download.