To install the plugins, you need to run sbot server, but if the plugins not there yet, sbot server will crash.
From memory, I think you can work around the problem of the server crashing if plugins are not there yet by doing this.
First manually install the plugins with sbot stopped:
cd ~/.ssb
npm i ssb-about
npm i ssb-backlinks
npm i ssb-fulltext
Then with sbot running, enable them instead of installing them:
sbot plugins.enable ssb-about
sbot plugins.enable ssb-backlinks
sbot plugins.enable ssb-fulltext
Then restart sbot.