Why can't I pre define symbols +, -, *, / as string and make use of them???? Can someone please help....

Question

If I use alphabet in place of above symbol as a representative of any operator the code seems to work perfectly..
But that is not the case in directly using symbols as operator..
https://code.sololearn.com/c6yCy0mc41E0/?ref=app
https://code.sololearn.com/c3w4kmYwxqps/?ref=app