Mono TLS1.2 issues - btls-cert-sync "command not found"
As of mono 4.8 beta 3, btls-cert-sync seems to have been merged into cert-sync.
Do make sure that you have an up-to-date version of cert-sync as it is possible to get old versions of cert-sync still installed even if you have updated mono because the ca-certificates-mono package has not been updated (Linux).
If you look at the cert-sync output you will see
Mono Certificate Store Sync - version 4.8.0.0...Importing into legacy system store:...Importing into BTLS system store:...