Hello!
Can you please specify what exactly attributes you are trying to add or remove? For example, if you want to remove from attribute set system attributes, it is not possible, since they are responsible for integrity of the system.
System attributes are marked with "stop" sign -
Here is also tutorial on attributes and their management -
https://www.mag-manager.com/useful-articles/magento-attribute-management/beginners-guide-understandi...