saturated operational amplifier

Hi all!
This is my first post so, sorry if I would not be proficient in discussing electronics or if maybe this is to demanding for the forum 😒

I am stuck in a practical experiment, I studied how amplifier for obtaining gain work. And I created a small circuit with an astable multivibrator, and operational, 2 resistors, two batteries of 9v:
Something like this:


Where the settings are :
non inverting input.
Rf = 300 K, Rg=150K
And the expected gain should be : Av= 1 + Rf/Rg = 3.
Doing a simulation I obtain reasonable results.
Where the output in volts of the operational is effectively augmented.
But when I build the real circuit it doesn't happen:
The oscillator generates a square wave that I can detect with
oscilloscope and even with a tester (between -4.80v +4.80v ), the output of the amplifier is instead stuck to 7.8v.
I have done this with a LM741, and also using a LM386 (one with
single supply) and nothing.
The crazy thing is that I obtain constant 7.8 v independently from the value of the feedback resistors (like they don't matter) and even if I completely disconnect the input.
So, it is like if whatever I do with this configuration

the amplifier saturates to 7.8v and end of the story.
I also tried the inverting input configuration and nothing.

Someone told me I need to insert a capacitor between the two part of the system and one in the output., but what is the logic in this ?
1- Why do I need a cap between the 555 and the op? and what should be its capacity ?

2- Putting a cap am I not interfiring in some kind of way into the frequency (it is a relative of an high pass in that way) ?

3- Why and what should be the capacity in the one in the output ?

4- Assuming i connect a photodiode with a transinpedence amplifier instead of the
square wave generator should I also put a cap before the amplifier ?

5- Since the LM386 is single supply if I use it instead of the LM741, should I add some kind of virtual ground in addition to the cap ?
Thank you for the patient 😁