Can't select users in the acknowledgement report? Then it's likely that the users you can't see don't have a home location. To explain why this matters, let's take a look at the main filter on that report:
Effectively, when you select one or more locations, the available users for the report will the the users who's home locations are those selected. This allows you to look at reports strictly and easily between locations.
If however there's a user or users that you can't see in the report, it will be because either a) those users don't have home locations or b) those users' home locations don't match the locations selected.
To add a home location to a user you will need to be an admin. To do so:
Go to Settings > Users
Click on the user that you cannot see in the report.
Next to the 'Home Location' heading, select their home location if they don't already have one.
Repeat for any other users missing from the report.
If adding a home location to your users does not resolve your problem, please don't hesitate to contact support.