MATLAB error on solving a system of equations : Error using sym.getEqn​sVars>chec​kVariables (line 92)

I try to solve a system of equations with 3 unknowns called "`

FISH_O_SYM(1,1),FISH_O_SYM(2,2),FISH_O_SYM(1,2)

`" (the element

FISH_O_SYM(2,1)

is not necessary since **I force the symmetry** to have `

FISH_O_SYM(1,2) = FISH_O_SYM(2,1)

`)

For the moment, by doing at the final :

sol = solve(eqn, [FISH_O_SYM(1,1),FISH_O_SYM(2,2),FISH_O_SYM(1,2)])

I get interesting solutions in the context of my work.

But now, given the fact all of this is about a matricial equation between 2x2 matrix, I mean I have a system of 4 equations with 3 unknowns, I would like to introduce in the second matrix (see below) another unknown `SIGMA_O = sigma_o` : this is a factor applied on each element f the `FISH_O_SYM` matrix. I do it like this :

 % Adding new observable "O" terms
    FISH_O_SYM = sym('o_', [2,2], 'real');
    SIGMA_O = sym('sigma_o', 'real');
    FISH_O_SYM = 1/(SIGMA_O*SIGMA_O) * (tril(FISH_O_SYM.') + triu(FISH_O_SYM,1));

but once I call the `solve` function :

sol = solve(eqn, [FISH_O_SYM(1,1),FISH_O_SYM(2,2),FISH_O_SYM(1,2),SIGMA_O])

The solver complains and generate the error message :

FISH_O_SYM =
    
    [ o_1_1/sigma_o^2, o_1_2/sigma_o^2]
    [ o_1_2/sigma_o^2, o_2_2/sigma_o^2]
    
    Error using sym.getEqnsVars>checkVariables (line 92)
    Second argument must be a vector of symbolic variables.
    
    Error in sym.getEqnsVars (line 56)
        checkVariables(vars);
    
    Error in sym/solve>getEqns (line 429)
    [eqns, vars] = sym.getEqnsVars(argv{:});
    
    Error in sym/solve (line 226)
    [eqns,vars,options] = getEqns(varargin{:});
    
    Error in solve_obs_O_1BIN_GOOD_but_with_issue_on_sigma_o_dev (line 66)
    sol = solve(eqn, [FISH_O_SYM(1,1),FISH_O_SYM(2,2),FISH_O_SYM(1,2),SIGMA_O])

I don't understand why he complains whereas in the first case, he succeed to solve the system of equations.

Any suggestion/track/clue/help is welcome to fix this issue.

Thanks in advance