JWT WriterToken function throwing IDX10653 Error

As the title, jwtSecurityTokenHandler.WriteToken(jwt) function is throwing IDX10653 Error. Full exception down there.

System.ArgumentOutOfRangeException: 'IDX10653: 
The encryption algorithm 'System.String' requires a key size of at least 'System.Int32' bits. 
Key 'Microsoft.IdentityModel.Tokens.SymmetricSecurityKey', is of size: 'System.Int32'. Arg_ParamName_Name

And my example usage is down there:

public AccessToken CreateToken(User user, List<OperationClaim> operationClaims)
{
    _accessTokenExpiration = DateTime.Now.AddMinutes(_tokenOptions.AccessTokenExpiration);
    var securityKey = SecurityKeyHelper.CreateSecurityKey(_tokenOptions.SecurityKey);
    var signingCredentials = SigningCredentialsHelper.CreateSigningCredentials(securityKey);
    var jwt = CreateJwtSecurityToken(_tokenOptions, user, signingCredentials, operationClaims);
    var jwtSecurityTokenHandler = new JwtSecurityTokenHandler();
    var token = jwtSecurityTokenHandler.WriteToken(jwt);

    return new AccessToken
    {
        Token = token,
        Expiration = _accessTokenExpiration
    };
}

I controlled my every variable and function. I think There is not a wrong thing and I don't find any solution for this error code. Any solution?

Edit: In the inner details an extra infos ->

Key 'Microsoft.IdentityModel.Tokens.SymmetricSecurityKey', is of size: 'System.Int32'. (Parameter 'key')

My SecurityKey creator function is like:

public static SecurityKey CreateSecurityKey(string securityKey)
{
    return new SymmetricSecurityKey(Encoding.UTF8.GetBytes(securityKey));
}