For clarification:
There is limited capacity within the Freeview spectrum, meaning that some 'SD' (Standard Definition) channels have to be accommodated on 'HD' (High Definition) multiplexes. (Multiplexes are the 'bundles' which Freeview channels are grouped into). Such stations can ONLY be received with 'HD' equipment (even though they're actually transmitting in 'SD').
The a516digital.com website (which is a standard reference source for those of us nerds who follow changes in TV broadcasting) states that Smithsonian is on Freeview channel 99 "for viewers with DVB-T2 compatible receivers, subject to local coverage"
https://www.a516digital.com/2019/02/smithsonian-channel-to-launch-across.html
Since 'DVB-T2' is the standard used by HD multiplexes, that means that 'HD' equipment is needed to receive the channel.
i.e. if your TV set (or set-top box) only has ordinary 'Freeview' built into it you WON'T be able to receive Smithsonian. You need a set (or box) with 'Freeview HD' built into it.