Apparently Ubuntu and its newer derivatives come with OpenSSL 3.0 and in this case Ruby versions earlier than 3.1 used OpenSSL 1.1

    In this case, the safest option is to follow the user manual and manually compile OpenSSL 1.1.

    It worked for me too with Pop!_OS and asdf

    PS: I put the export inside my .zshrc

Install the dependencies:
```bash
sudo apt install build-essential checkinstall zlib1g-dev
```
Download OpenSSL 1.1.1:
```bash
cd ~/Downloads
wget https://www.openssl.org/source/openssl-1.1.1n.tar.gz
tar xf openssl-1.1.1n.tar.gz
```
Compile it:
```bash
cd ~/Downloads/openssl-1.1.1n
./config --prefix=/opt/openssl-1.1.1n --openssldir=/opt/openssl-1.1.1n shared zlib
make
make test
sudo make install
```
Link the system's certs to OpenSSL 1.1.1 directory:
```bash
sudo rm -rf /opt/openssl-1.1.1n/certs
sudo ln -s /etc/ssl/certs /opt/openssl-1.1.1n
```
Use RUBY_CONFIGURE_OPTS=--with-openssl-dir=/opt/openssl-1.1.1n before the command to install the ruby version:
```bash
RUBY_CONFIGURE_OPTS=--with-openssl-dir=/opt/openssl-1.1.1n rbenv install 2.7.6
```
If you want to make this permanent, add this line to you .bashrc or .zshrc file:
```bash
export RUBY_CONFIGURE_OPTS="--with-openssl-dir=/opt/openssl-1.1.1n/"
```
Then you don't need to use RUBY_CONFIGURE_OPTS=--with-openssl-dir=/opt/openssl-1.1.1n before the command anymore.

Font: https://github.com/rbenv/ruby-build/discussions/1940#discussioncomment-2663209