Inside of AppWorld, I can find no app named "Myfitnesspal"...are you sure that is the exact name? In any event, you should look at the AppWorld catalog page. Each app has several requirements for compatibility including device model number, OS level, carrier, country, and perhaps others...you should review all of those to ensure that you meet them all.
Occam's Razor nearly always applies when troubleshooting technology issues!
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