bigmanik is correct, GnuPG is unable to find the data to which the signature file pertains. Bitcoin core has a two-step verification process; first you need to download three files; the binary (.zip) for your version of MacOS, the SHA256 binary hashes, and SHA256 binary signature. The binary hashes file will not have a file extension, but it can be opened and read by any text editor. The signature file (it looks like you've already downloaded that) will have the .asc file extension. Download all the files to the same directory, then open a terminal window in that directory.
The first step; use GPG to verify the hash file matches the data signed by the signatures in the signature file.
gpg --verify SHA256SUMS.asc
The second step; use the MacOS built in sha tool to very the hash of the .zip file.
shasum -a 256 --ignore-missing --check SHA256SUMS
More detailed instructions can be found here:
https://bitcoincore.org/en/download/