You need to wrap each of the values you are comparing op to in double-quotes:
For example:
if (op == -)
has to be:
if (op == "-")
That tells the compiler that you are comparing the op string variable to the double-quoted string constant.
With your code, the compiler thinks you are attempting to subtract something from something else -- thus the "illegal start of expression" as in a subtraction expression.
You'll need to fix all of those comparison statements.
And to compare strings you should not use the == in Java.
Instead you should use the equals method of the string object.
Each of your statements should look like the following (obviously replacing the character you are attempting to check for):
if (op.equals("-"))
The Importance of Examining Error Messages
Also as you're learning to code it is really important to examine the errors you get very closely and then when you are looking for help to reproduce those exact errors when explaining them to others who may be of help.
Here's what I mean. I reproduced your error and let's look closely at what the error looks like:
C:\dev\tools\JavaProjects>..\JavaJDK17\bin\javac first.java
first.java:4: error: illegal start of expression
if (op == -){
^
1 error
the first line is where I ran the javac (java compiler).
The second line gives you the exact line where the error occurred in the source file (first.java) line 4 -- see the :4? That's what that means.
Also notice there is a caret ^ pointing up at the location where the compiler thinks the problem occurred. We know it is near there so it does begin to help you guess.
Including that kind of error information makes it far easier for someone else to help you find the problem.