Microsoft confirmed that some users are getting error 0x800f081f while installing KB5000842.

This will not affect all users, but some! However, it seems that every update released to fix something in a previous update, another update is needed to fix bugs in the fix. So it seems we are always one step behind. Waiting for the next update.

Microsoft is aware of the issue, and is planning to resolve this in a new Cumulative Update for 21H1. Some users claimed that the problem was fixed by doing an in-place upgrade to repair this.

Source: blog.Windows