Edit groups and category discount

[LightBlue82]

Hello, for some reason I can't add category discounts to my groups. Creating the group works, but as soon as I select and save a category and discount, nothing happens. Can anyone help me out here or explain how I can create different discount rates for a single group?

kind Regards
LB

[Prestashop Addict]

Please give more context, PS and php version

[LightBlue82]

I work with PrestaShop 1.6.1
PHP version 8.1.29 (supports 64bit values)

I open my groups and go to edit then I click on add category discount/Kategorie-Rabatt (I leave discount/Rabatt empty because I want to add two or more categories separately):

Then this window opens:

I can enter the discount but as soon as I select any category (no matter if parent-folder or sub-folders. I can easily open all subfolders) the whole field closes without me being able to click save and no category discount has been added.

[Prestashop Addict]

PS 1.6.1 and PHP 8.1, are you sure, not compatible!

[LightBlue82]

this is what the server info on my wordpress site says. could prestashop work with a different php setting?

I am new here, i understand and can work with the basics alsi i am good at finding logical connections and understanding how things work.
But I don't really understand much about coding and have no idea about PHP and modules or which modules are important or even compatible together. 

for instance:  this M4 (Discount and Surcharge) module is installed twice with small differences. what happens if I delete one is that even possible?

I just know it's a bit frustrating, I often have to edit and save pages 12 to 16 times for it to accept changes in the store, some things don't seem to work at all and some things are confusing (two identical modules,...)

In short, if you don't mind me asking:
What do you think would be my best option to get Prestashop working again so that my modules and everything else works smoothly?