Unless you have a specific reason to be on macOS 10.15 still, you should be able to upgrade to macOS for free in Software Update in System Preferences.
If your Mac is too old for this, you can upgrade to macOS 11 (or newer) by grabbing an Installer from a working Mac or the Mr. Macintosh page and from there, downloading, building (it does this for you automatically), and installing OpenCore Legacy Patcher to a 32 GB USB flash drive. Then restart from the OCLP EFI environment, install macOS normally, and install the patcher itself and the post install patches to the internal startup disk (your SSD) afterwards — and I will definitely say that you will need an SSD or NVMe to run anything beyond macOS 10.x, so only upgrade if your Mac has flash storage in it or it’ll be really slow.
Hope this helps! And if you are stuck on Catalina or need it, you can still install a port of qemu on it (but beware it’ll run without Apple’s Hypervisor acceleration, which means it’ll be slow). Another option would be to delete the new VirtualBox and install an older version. If this doesn’t work, you’ll probably need to delete any helpers and kernel extensions (kexts) that were installed with the old version. Haiku works well for me on VBox before 7.x; I will say some lag for running Haiku in VBox has always been there (for example, when compiling a Haiku image, and this looks like it’s due to the way VirtualBox seems to interpret calls, far as I know) but it seems like the Haiku lag went up (at least for me) with VBox 7 on both Gnu/Linux and macOS. In any case, hope this helps and happy virtualboxing!