I am unable to download SXTTools package in Julia.
Not aware of a SXTTools package for Julia… GitHub - jaspershen/sxtTools: Small tools of SXT is a package for R. There is GitHub - gulabd/SXTMerger.jl: A tool to merge AstroSat/SXT event lists from different orbits but not really sure what you are looking for.
In any case you can always add an unregistered package by adding
] add https://link.to.repo/SXTTools.jl.git or something like that.
Also, you might want to update your Julia version