No project has a win-64-bit application yet, or if they have, they're using wrong platform-name so isn't compatible with BOINC-client.
BOINC-clients can now also use an "alt_platform_name" so can also use 32-bit applications, but due to a small bug in v5.9.4 the alt name is the same as primary... So, you'll have to wait for v5.9.5.
BTW, also remember, if you upgrades old 32-bit to 64-bit, you'll reset all projects...