Can't build due to missing OpenJDK

When trying to build in Builder I get this error error: Requested extension org.freedesktop.Sdk.Extension.openjdk11 not installed. However, when I run flatpak install flathub org.freedesktop.Sdk.Extension.openjdk11 it just says Skipping: org.freedesktop.Sdk.Extension.openjdk11/x86_64/18.08 is already installed (because I had already installed it earlier). Somehow it being available doesn't fix the build it seems.