If you have accidentally answered “No” when asked whether you want to allow Threema to access your photos, you can grant permission as here:
iOS Settings > Privacy & Security > Photos > Threema
If Threema is not listed in the location mentioned above, please remove restrictions for Threema:
iOS 12: iOS Settings > Screen Time > Restrictions > Photos > Allow Changes
iOS 11 and earlier: iOS Settings > General > Restrictions > Photos > Allow Changes
