I tried to sign a message from gui v18 and the debug console without success: With the popup it shows red text:

"the entered address does not refer to a key. Please check the address and try again"

With the console:

"Address does not refer to key (code -3)"

the approach was:
- get a fresh public address from the wallet (new receive address)
- paste it into the address field
- type 'foobar' into the message field
- click sign message

in the console i unlocked the wallet (for long enough!) then
`signmessage "pupaddr" "foobar"

both with no success. I double checked the pubaddr (taken from the address field on the right button).

the throw happens here in the sourcecode. The addr. starts with 3... .