In macOS Terminal, can you run the following two commands post their output. If Action -> Download works, but BCA gives you this error message, it may be an unstable network connection.
Here are examples of each command and the corresponding output.
ping -c3 swcdn.apple.com
PING swcdn.g.aaplimg.com (17.253.97.201): 56 data bytes
64 bytes from 17.253.97.201: icmp_seq=0 ttl=60 time=10.885 ms
64 bytes from 17.253.97.201: icmp_seq=1 ttl=60 time=10.880 ms
64 bytes from 17.253.97.201: icmp_seq=2 ttl=60 time=10.100 ms
--- swcdn.g.aaplimg.com ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
dig swcdn.apple.com
; <<>> DiG 9.10.6 <<>> swcdn.apple.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 51366
;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;swcdn.apple.com. IN A
;; ANSWER SECTION:
swcdn.apple.com. 19215 IN CNAME swcdn.apple.com.akadns.net.
swcdn.apple.com.akadns.net. 247 IN CNAME swcdn.g.aaplimg.com.
swcdn.g.aaplimg.com. 15 IN A 17.253.97.202
swcdn.g.aaplimg.com. 15 IN A 17.253.97.206
;; Query time: 7 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Thu Oct 07 23:24:27 EDT 2021
;; MSG SIZE rcvd: 146