Update: If you are using Add-on Updater 21.05 with NVDA versions earlier than 2021.1 beta 1 or alpha-22877, you can now download Add-on Updater 21.05.1 via Add-on Updater. After that, you can enable update checks.
Specifically, follow the below steps as exactly as shown:
If you are not using NVDA alpha.22877 or 2021.1 beta 1 or later and would like to use one of these:
If you are using NVDA alpha.22877 or 2021.1 beta 1 or later and happen to be using Add-on Updater 21.05: you must download Add-on Updater 21.05.1 manually (see the earlier message for download link).
If you are using Add-on Updater 21.05.1: you can ignore this as you are using latest release.
Expect more messages from add-on authors as add-on updates declaring support for NvDA 2021.1 beta 1 are released (some of them include mine).