The following commands worked perfectly on my Ubuntu 20.04. It should work for 18.04 too:
Enabling the Apache CouchDB package repository
sudo apt update && sudo apt install -y curl apt-transport-https gnupg
curl https://couchdb.apache.org/repo/keys.asc | gpg --dearmor | sudo
tee /usr/share/keyrings/couchdb-archive-keyring.gpg >/dev/null 2>&1
source /etc/os-release
echo "deb [signed-by=/usr/share/keyrings/couchdb-archive-keyring.gpg] https://apache.jfrog.io/artifactory/couchdb-deb/ ${VERSION_CODENAME} main" \
| sudo tee /etc/apt/sources.list.d/couchdb.list >/dev/null
Installing the Apache CouchDB packages
sudo apt update
sudo apt install -y couchdb