These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.59 s (backtrack: 0/20).

[ebuild  N     ] acct-group/lxc-0-r2::gentoo  0 KiB
[ebuild     U  ] sys-libs/libcap-2.78::gentoo [2.77::gentoo] USE="pam static-libs* -tools" 197 KiB
[ebuild  N     ] acct-user/lxc-0-r2::gentoo  0 KiB
[ebuild  N     ] app-containers/lxc-7.0.0-r1:0/1.700::gentoo  USE="caps pam seccomp ssl tools -apparmor -doc -examples -io-uring -landlock -man (-selinux) (-systemd) -test -test-install -verify-sig" 941 KiB

Total: 4 packages (1 upgrade, 3 new), Size of downloads: 1137 KiB

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by app-containers/lxc-7.0.0-r1::gentoo[tools,caps]
# required by =app-containers/lxc-7.0.0-r1 (argument)
>=sys-libs/libcap-2.78 static-libs

!!! All ebuilds that could satisfy "sys-libs/libcap[static-libs]" have been masked.
!!! One of the following masked packages is required to complete your request:
- sys-libs/libcap-9999::gentoo (masked by: missing keyword)
- sys-libs/libcap-2.78::gentoo (masked by: )

(dependency required by "app-containers/lxc-7.0.0-r1::gentoo[tools,caps]" [ebuild])
(dependency required by "=app-containers/lxc-7.0.0-r1" [argument])
For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.