First: Using --force is kind of a last resort; I don't see how adding that to the command above improves your situation.
[you can't force it to read the private key]
Let's have a look at which certs are currently maintained by acme.sh with: ./acme.sh --list
Your private key file is missing or not accessible?
Check in the path where your cert file for this domain is kept (as per your nginx cert config).
If the private .key file is indeed missing I wonder if you might be best to remove this configuration and start again, alternatively create a new private key file (look where the rest of your cert files are being created) or copy a different one.
It might not be relevant in this specific case, but removing previously issued certificates carelessly by removing entire directories from an ACME client, such as in this case, might lead to rate limit issues. I'm not saying you'd do that in the future, but I thought it might be a good idea to warn you and/or other people finding this thread anyway