Snapd error Method Not Allowed

Hi there…
i try to install nextcloud but got these error

image

anyone can help me?

snap version

root@ubuntu1804:/etc# snap version
snap    2.40+18.04
snapd   2.40+18.04
series  16
ubuntu  18.04
kernel  4.15.0-64-generic

snapd log

root@ubuntu1804:/etc# journalctl -f -u snapd
-- Logs begin at Wed 2019-09-25 15:18:47 WIB. --
Sep 26 10:59:07 ubuntu1804 snapd[12486]: stateengine.go:108: state ensure error: cannot refresh snap-declaration for "core": Get https://api.snapcraft.io/api/v1/snaps/assertions/snap-declaration/16/99T7MUlRhtI3U0QFgl5mXXESAiSwt776?max-format=3: Method Not Allowed
Sep 26 11:14:59 ubuntu1804 systemd[1]: Stopping Snappy daemon...
Sep 26 11:14:59 ubuntu1804 snapd[12486]: main.go:166: Exiting on terminated signal.
Sep 26 11:14:59 ubuntu1804 systemd[1]: Stopped Snappy daemon.
Sep 26 11:14:59 ubuntu1804 systemd[1]: Starting Snappy daemon...
Sep 26 11:14:59 ubuntu1804 snapd[12986]: AppArmor status: apparmor is enabled and all features are available
Sep 26 11:14:59 ubuntu1804 snapd[12986]: daemon.go:338: started snapd/2.40+18.04 (series 16; classic) ubuntu/18.04 (amd64) linux/4.15.0-64-generic.
Sep 26 11:14:59 ubuntu1804 systemd[1]: Started Snappy daemon.
Sep 26 11:14:59 ubuntu1804 snapd[12986]: stateengine.go:108: state ensure error: Get https://api.snapcraft.io/api/v1/snaps/sections: Method Not Allowed
Sep 26 11:25:29 ubuntu1804 snapd[12986]: api.go:950: Installing snap "nextcloud" revision unset