Can't install app (Error code: -26) I am trying to install Lost Android and Wheres My Droid but I always get this error on both apps:



I have sufficient free space and have installed other apps so that's not an issue. I am using custom ROM MIUI 4.4.18.

I found this page having similar issue. They suggest manual apk install which works but updating the apk results to the same error on Google play.
That seems to be a MIUI-specific error. From the Locus Map help (translated by me):


  This error occurs with Xiaomi devices, but has nothing to do with Google Play Store. Error -26 is caused by MIUI's package manager and stands for INSTALL_FAILED_SYSTEM_INCOMPATIBLE. Which means: for some unknown reason MIUI thinks the app is incompatible, and refuses to install it.
  
  Solutions:
  
  
  Try to install an older version of MIUI which is compatible with the app and doesn't block the installation.
  Then disable automatic updates Aktualisierung of MIUI
  Alternatively just wait until there's a newer, bug-fixed version of MIUI
  


Also, additional to your find, there's a thread on MIUI forum, which – surprise! – says almost the same. Dated 2014 (ouch).



Funnily, the same error -26 is also reported for the Samsung Galaxy S3 on Sprint. Folks here seem to have been more lucky. One user reports the following procedure as being successful:


  Go to Application manager and find Google Play Store, Erase the cache and data. Then go to Accounts and remove the account associated with the google play services. Restart your phone phone and add the account back.




So if the latter doesn't work, I'm afraid you've to check that with MIUI. I wonder if it really takes them 2 years to get it fixed.

PS: What strangely seems to work is opening the downloaded .apk file using a file manager. Though that should just be an interface to the pm install command (which is also used indirectly by adb install) which did not work out when used directly.