Cannot update to Ubuntu 21.04
Solution 1:
Correct.
See the 21.04 Release Notes:
Release Upgrades
Upgrades from Ubuntu 20.10 to Ubuntu 21.04 are not enabled as it is possible for some systems to end up in an unbootable state if they use EFI version 1.10 - bug 1925010. Release upgrades will be enabled once an updated version of shim is available which is compatible with EFI version 1.10.