Hi @DirkEyfrig,
PUT /rest/api/content/{pageId}/restriction is only available in Confluence 8.8 and newer.
If you are using an older Version of Confluence you can use the experimental version PUT /rest/experimental/content/{pageId}/restriction instead. This one uses the same body format and has been around since Confluence 6. It is also still available in Confluence 9, in case you want to support multiple Confluence versions.