Is it permissible to take out a loan from the bank against the guarantee of a retirement pension to undergo surgery, knowing that the bank charges an 8% interest rate and there is no other way to cover the cost of the surgery?

The first duty of one in dire straits is to sincerely seek refuge in Allah Almighty. This is the greatest stance in affliction, and Allah has praised Himself for relieving the distress of the afflicted and condemned those heedless of this stance. So, direct your complaints to Allah, seek His help, and rely upon Him.

Allah has promised the righteous an exit from difficulty and provision from where they do not expect.

As for taking a loan guaranteed by salary, it is explicit usury. Some scholars have permitted it in cases of necessity where there is a fear of perishing or severe harm, or unbearable hardship. However, need does not permit what is forbidden.

Therefore, this loan is not permissible for you unless your situation reaches the level of dire necessity, and you are overwhelmingly certain of its benefit, and it is not possible to delay it until an interest-free loan becomes available to you.