Dotted Rests

I would like to know how to insert a dotted rest in Dorico SE

Welcome to the forum, Tax-Free. Dorico will automatically create a dotted rest as appropriate according to the options defined in Notation Options (which of course you can’t change in Dorico SE, I’m afraid). But you can force a dotted rest anywhere by hitting the O key to activate Force Duratoins, then input the rest.