The main issue with AWS lightsail on Bitnami is the get to correct root folder and executing the commands with right permission.
Once you login via SSH to Bitnami AWS Lightsail the path need to be changed to magento root folder.
The the owner ship for the folder need to be changed for daemon user.
sudo chown -R daemon:daemon /opt/bitnami/apps/magento/htdocs
Now you can try the command to clean cache
sudo bin/magento-cli cache:clean
Then the themes need to be downloaded and copied via FTP client to AWS lightsail.
AWS lightsail supports SFTP. You need to download the private key from AWS lightsail and update on the FTP client.
Once the connection to AWS is established. Copy the the files to the root folder Magento as per the instructions from theme supplier.
sudo bin/magento-cli module:enable <theme>_Base <Add modules one after another>
Then you need to upgrade for all the new and existing modules.
sudo bin/magento-cli setup:upgrade
Then setup static content as usual.
sudo bin/magento-cli setup:static-content:deploy
Test the theme.