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The reason for this incompatibility is the permanent development of Locus application and the Android itself as well. Some functions simply cannot be developed for older Android devices.
The solution is to install the latest Locus Map version that supports your version of Android.
If you have not purchased Locus Map Classic/Pro license yet, please do so in the Google Play Store in an Android emulator on your PC, running Android higher than 5.0 (e.g. Bluestacks) - the license validation will work for the old Locus Map version as well.