“This looks like a USB‑C charging cable, or the cable is in a port without the ⚡︎ symbol.”
Use a Thunderbolt cable, in a port with the ⚡︎ symbol on both Macs: not every USB‑C port has Thunderbolt. Meanwhile the picture can continue over the cable’s USB link, compressed. A USB‑C cable without Thunderbolt
“Thunderbolt didn’t connect. Try a ⚡︎ port; the cable may be a USB‑C charging cable.”
The menu’s version of the same message. When it starts with “On USB for now”, the picture already goes over the cable’s USB link.
“This port doesn’t support Thunderbolt.”
Move the cable to a port with the ⚡︎ symbol.
“Thunderbolt Bridge is turned off”
macOS joins two Macs over Thunderbolt with a network service called Thunderbolt Bridge. Turn it on in System Settings › Network; the button Open Network Settings… takes you there.
“Thunderbolt Bridge isn’t set up”
The service was removed. Add the Thunderbolt Bridge service again in System Settings › Network.
“Allow the accessory on this Mac”
A Mac laptop asks whether the other Mac may connect. Click Allow when macOS asks.
“The cable connection is getting ready…”
Give it a few seconds.
“This Mac has no Thunderbolt”
Connect the Macs over Wi‑Fi or Ethernet instead. Use Wi‑Fi or Ethernet