In general, object storage solutions like Spaces are not meant to be accessed like a file system. They are API driven and most often used programmatically. Check out this tutorial to learn more about object storage and if it's right for your use case.

For managing Spaces from the desktop, there are a number of GUI tools. One popular option is Cyberduck:

• How to Manage DigitalOcean Spaces with Cyberduck

If your use case really requires mounting your Space like a local filesystem, s3fs-fuse is probably the tool you are looking for. On Ubuntu, you can install it with:

sudo apt install s3fs

You'll also need to get your Spaces access key and secret key (see here).
Then add those credentials to ~/.passwd-s3fs, create the local directory, and mount your Space:

echo $SPACES_KEY:$SPACES_SECRET > ~/.passwd-s3fs
mkdir -p /path/to/local/directory
s3fs your-space /path/to/local/directory -ourl=https://nyc3.digitaloceanspaces.com -ouse_cache=/tmp

I have a question, once is mounted, why I can't explore my directories in spaces, basically, I only can work in spaces root, any suggestion?