Google Play Store often limits the access to some apps based on the software/hardware specifications of your device and your location. Although we do not limit the access to the Check-in apps to any audiences in particular and we are doing as best as we can to make Check-in app compatible with vast majority of devices and Android versions, you can still encounter this issue due to Google's limitations.
However, the good news is that you can still use this app but you will need to side load it instead of installing it through the Google Play Store.
Sideloading of the Android app is considered as risky but if you are using it for the app equal with the one available on the Google Play Store, there is nothing to worry about. To do this, you will first need to visit Settings area of your device, head to Security category and then choose Device. Among all the options which you should leave as-is, find Unknown Sources item and flick the switch or check the checkbox to allow installation from the unknown sources. This will cause a pop-up to appear to inform you that what you are doing now is considered risky, but just tap OK and exit from settings.
Next, download Check-in app installation file from here and transfer it to your device. Once transferred, tap on that file and it will begin the installation process. Once you have finished installation, head back to the Settings -> Security -> Device and uncheck Unknown Sources to revert settings back to the safety zone.