I am unable to download SXTTools package in Julia.
Not aware of a SXTTools package for Julia… https://github.com/jaspershen/sxtTools/ is a package for R. There is https://github.com/gulabd/SXTMerger.jl 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