This has been asked quite a few times (most unanswered), e.g.
- Indent/unindent currently only works with lists
- Increase indentation (without using bulleted/numbered list)
- Error "indent / unindent currently only works with list"
In short: There seems to be an addon that could help but I don’t know which one it is. Click through those links to see some workarounds, e.g. the one I mentioned here: Error "indent / unindent currently only works with list" - #2 by Anon_0000.
Edit: Maybe it’s this addon by @glutanimate?
https://ankiweb.net/shared/info/295889520