What happened
The App Store blocked the purchase for this device or Apple Account. HomeMapper can't change that, and trying again gives the same result. Apple doesn't charge you for a purchase it blocks.
The App Store doesn't show why purchases are off, only that they are. The usual causes are below, with what to do about each.
iPhone and iPad: Screen Time
Screen Time can turn off in-app purchases. Go to Settings → Screen Time → Apps & Websites → App Store → In-app Purchases. If it shows Blocked, change it to Allowed.
On older versions of iOS and iPadOS the same setting is under Settings → Screen Time → Content & Privacy Restrictions → iTunes & App Store Purchases → In-app Purchases. If it shows Don't Allow, change it to Allow.
Apple's guide to Screen Time restrictions on iPhone and iPad
Mac
On a Mac, go to System Settings → Screen Time → Content & Privacy → Store Restrictions. Under Allowed on iOS, turn on In-app Purchases. Apple describes these switches as applying to iPhone and iPad, and HomeMapper on a Mac is an iPad app, so this is the switch to check.
A child account in Family Sharing
On a child's account, the family organizer controls purchases. Ask them to change the setting.
If the family uses Ask to Buy, a purchase sends the organizer a request instead of being blocked. HomeMapper shows that as waiting for approval.
A work or school device
If an organization manages this device, it controls whether purchases are allowed. Ask whoever looks after the device there.
Apple's list of restrictions an organization can set on iPhone and iPad
None of these
If none of the above applies, the restriction is on your Apple Account rather than this device. Apple Support can look at the account and remove it.