Smart Switch Connection Issues
This looks like a switch loop where the hot is the white wire the switched hot going back on the red and black. Very common to do this at the initial wire pull for a fan and a light. You can get a neutral to this location but it will require moving wires around and eliminating 1 switch (you loose the fan option to gain smart switch).
The white should have been reidentified as a hot usually with a black permanent marker or tape. This step is regularly overlooked and homeowners not knowing this causing miswires, it will again be a neutral so don’t fix it now.
With the breaker off.
The wiring would need to be changed in the ceiling from this cable.
the white wire now connected to black would be moved to the other group of white wires now the white down in the switch box is neutral.
The black going to the light or not used goes to the black where the white was usually a black this goes Down in the switch box this is your hot or line now.
Place the red on the other side of the same switch the black wire is on, this is now the switched hot going back on the red to the black of the light  (red wire to black of light).
Turn the power back on and it should work, just the 1 switch now convert it to your smart switch.
Get a double gang cover plate with one side blanked off or put the switch in knowing it is not connected to anything.