A usable Let's Encrypt certificate consists of a public and private key pair, on Windows this is conventionally packaged in a PFX file (also known as a PKCS12 container format, or .p12), on other operating systems and services these are often split into a few different files. There is nothing special about the certificate itself which stops you working with it or converting it in various ways, but there is something specific about the way the cert and private key have been imported into the (protected) certificate store.
win-acme will output a bunch of files and as part of the default process it will attempt to store the PFX in the local machine certificate store, under Web Hosting (this is it's default, the Personal/My store is also fine, it just has to be under Local Machine, not a specific user). It will use a random password by default so you will probably want to specify one instead.
Your process is falling down because you are manually exporting the certificate and losing some information. If you can't export your private key then you cannot use the certificate (it's just a public certificate/key at that point, not one you can use).
I'd suggest you use the win-acme option to export to a PFX file (PFX File Plugin) and set the export location and password win-acme alternatively mark the private key as exportable in your win-acme settings:
- [win-acme](the associated private key is marked as not exportable · Issue #1373 · win-acme/win-acme · GitHub see also win-acme)
- the associated private key is marked as not exportable · Issue #1373 · win-acme/win-acme · GitHub
- see also https://www.win-acme.com/manual/advanced-use/load-balancing
For general win-acme help you should use their GitHub discussions: Discussions · win-acme/win-acme · GitHub