Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
With Linux, you can make use of the mkdir command to make new directories.
Also please note that the command snippets where you got the ~/.secrets/certbot from are just EXAMPLES as clearly stated at the top of the examples. So that means the commands are not necessarily a 1-on-1 fit for your specific situation.
I assumed from the instructions that it should exist. There is no indication of creating the directory, just the digitalocean.ini file within the directory.
The command snippets are just examples. Documentation shouldn't need to spell out every small (and IMO obvious) detail.
As they're just examples, you're free to put the digitalocean.ini anywhere you want, as long as it's SECURE (i.e.: root readable and writable only). Heck, you don't even need to name it digitalocean.ini, you could name it bert_and_ernie.ini for all I care