after I open megasync ubuntu 20.04.deb with Gdebi but it comes the error that i have at first time... Error: the dipendencies cannot be satisfied: Megasync
If you ONLY install FlatPak, that would be OK.
But you also tried to install Nautilus integration (which is available in .deb only).
FlatPak and SNAP is running in a so-called sandbox environment for security reasons. Did you notice that you did not need sudo to install/uninstall FlatPak app?
The problem is that Nautilus integration app needs to have dependency files from Mega desktop app. Due to its sandbox nature, FlatPak would not allow the Nautilus installer to access those dependency files. Hence you saw the error message during the installation.