Firstly, Raspberry Pi's should generally be avoided for running a node as they don't have hardware AES, which means you'll have slower syncing/scanning/verification. There are many comparable devices which do have hardware AES, thus are better options.
Second, if you are definitely running a 64-bit version of Ubuntu (see
uname -a to confirm) and are using a board the has hardware AES (which Raspberry Pi's don't), you can use the ARMv8 release.
Otherwise (and in your case) use the ARMv7 release. If you're really brave / patient (it takes a long time on Raspberry Pi's), compile from source.