Empty sym: 0-by-1

I want to change C*m*(Co-Ci) = U*A*((Hi-Ci)-(Ho-Co))/log((Hi-Ci)/(Ho-Co)) to Co = ~~~~~

but The following error occurs. I spent over a week

If I input a number, the result comes out, but why doesn't the result come out when I enter a variable?

syms C m Co Ci Hi Ho U A
eqn = C*m*(Co-Ci) == U*A*((Hi-Ci)-(Ho-Co))/log((Hi-Ci)/(Ho-Co))

eqn = 

S=solve(eqn,Ci,"Real",true)

Warning: Unable to find explicit solution. For options, see help.

 
S =
 
Empty sym: 0-by-1
 

[Walter Roberson]

What leads you to expect that a closed-form solution exists?