When you open the community via https://lemmy.ml/c/librewolf you are using the lemmy.ml instance. Since your account is on slrpnk.net you need to open the community using this instance to subscribe. So:
- Open slrpnk.net
- Go to "Communities"
- Search for "librewolf" and make sure you have "ALL" selected
- Go to the community using the search results
- Then subscribe