It is a good idea to uninstall all apps do not use on your device to free memory and prevent it from slowing down. However, you may not be able to get rid of some of them and it may be due to two reasons: it is an app that comes from the factory or is a device manager.
Normally, uninstalling an application on Android is easy. You must go to Settings> Applications, choose the desired app and Touchez on the "Uninstall" button.
If instead of "Uninstall" there is only the "Disable" button, then it is an app by default. In some brands like Samsung, that is the case of Whatsapp, Facebook or Instagram. If the button is disabled, you may be a device administrator.
Case 1: It is a factory application
There is no way to uninstall these types of applications, unless you have root permissions on the device. If this is your case, you will only have to install a tool with the ability to eliminate these types of apps, such as Titanium Backup.
I do not recommend uninstalling apps this way unless you know what you are doing. In addition to rooting your computer, if you delete a critical app from the system, the phone may become unstable or will not start the next time you start it.
Instead of the "Uninstall" button, preinstalled applications like WhatsApp, Facebook or Instagram They usually offer a "Disable" button that has virtually the same effect as uninstalling. The only difference is that deactivating it will take up storage space, but will be "dead" in the system.
This process is generally harmless, since only secure apps to disable have this possibility. However, if you see any irregularities, you can re-activate them with the same ease.
Case 2: You are a device administrator
Some applications need to be configured as a device administrator so that they can perform certain privileged actions. For example, set passwords, lock and unlock the screen or get the location of the phone. Examples of these applications are Cerberus or Greenify.
The Uninstall button of these applications will not work as long as they are not removed from the "device administrators" list.
To remove an app from the system administrator privileges, the path to follow is usually Settings> Security> Device administrators. There uncheck the app and confirm the action. After that you can uninstall it in a conventional way or even delete the data and force its closure, actions that are also usually blocked in the previous state.
In very particular cases, the application may be hidden within this list of device administrators. In that case you must first discover that hidden device manager and then disable and uninstall it.