the plus button doesn’t show when i press the home screen

Presumably you are attempting to add Widgets to your Home Screen or Today view? If so, while the Widgets and layout differ between iPadOS14 and iPadOS15, the method of adding widgets is very similar…

For iPadOS14, from your first Home Screen, simply touch any area of empty space on screen (avoiding the corners or extreme edges) - and hold - until the screen icons “jiggle”. The “+” icon will simultaneously appear at top left of the screen.

For iPadOS15, the process to add widgets is identical - except that you can add Widgets to any of your Home screens.

More information can be found here:

Use widgets on your iPad - Apple Support

Add widgets on iPad - Apple Support

Is your iPad updated to iPadOS 15.4.1, the current iPadOS? If not, that would be one reason it's not working. Your tag line suggests you're on iPadOS 14.