I can only see 3 modes in my fx - 991ES. how can i unlock the rest of the modes?

The NEWYES NY-991MS and CTIFREE fx-991ES PLUS calculators do not have a dedicated "FACT" button like the Casio FX-83GTX. However, you can still find prime numbers on these calculators.
To find prime numbers:
NEWYES NY-991MS:
Press the "MODE" button.
Choose "PRIME" from the list of options.
Enter the desired number and press "=" to check if it's prime.
CTIFREE fx-991ES PLUS:
Press the "MODE" button.
Select "TABLE" or "TABLESET."
Input a starting point and an ending point, then observe the table for prime numbers.
Although the techniques may vary from those used on the Casio FX-83GTX, they prove to be effective for prime number calculations on the designated calculators.

The Casio fx-991ES calculator cannot perform 7x7 matrix operations.

Press MODE then 1. This will get you out of about any other mode.

PDF]fx-570MS/991MS Additional Functions - Support - Casio

support.casio.com/pdf/004/fx570MS_991MS_E.pdf

Casiodisplay format (Disp) settings while the calculator is in the BASE ... Be sure to checkthe current calculation mode (SD, REG,. COMP ... Use [ and ] to display the expression 4 + 4. Press A ... want to perform scientific function calculations. COMP .

The Casio fx-991es, fx-570es and - Numericana

www.numericana.com/answer/casio.htm

The Casio fx-991ex succeeds the fx-991es (functionally identical to the ... Both are nonprogrammable nongraphing scientific calculators, with natural ... Side-by-side comparison of the fx-991EX and fx-991ES PLUS. ... Displays QR codes for smartphone use .... Work out a formula for many values of the input variables.

CASIO fx-991MS Calculator Tutorial: Lesson 5- Dealing with ...

Press [SHIFT][ division key]. It is marked with nCr.

You should try to explore the calculator to find its capabilities. You can also read the manual. Everything is in it.
Press the Mode button, then the down arrow key. On the second page of the mode list you find 2:Verif

On a fx-991MS? With many other models, including the fx-991ES, I'd say you have the calculator in binary mode but the 991MS doesn't have such a mode.

In table Mode you can enter the formula for a function and generate a table of values. For a 991ES Plus you can enter two functions. However if you change MODE everything will be erased.
In EQN (equation) there is a certain number of built in formula types you can choose. You will be asked for the coefficients which will be used to solve the equation selected. However if you change MODE your lose what you entered. So basically you cannot save formulas to use as a memory aid (if that is what your intention is.) Sorry.

MthIO is the default. Press SHIFT MODE 2 to select LineIO mode. All of the settings can be accessed through either MODE or SHIFT MODE.

Hello,
Just in case : Press [SHIFT][CLR] (9) [3: ALL] to reset the calculator and try [MODE] again.
If that does not work, your calculator may be defective or a fake (not the genuine article). If at all possible, return it to where you bought it and get a refund.

Here is a picture of the FX-115ES, its face looks like the 991ES. Hope I am mistaken.