People outside of countries where the Echo is a supported product (countries such as US and UK support it) have had issues downloading Alexa app to their phones or tablets. Try using the web browser access at alexa.amazon.com
Also contact Echo Support at this number:
You can reach Amazon Echo Customer Support by phone directly and toll-free at 1-877-375-9365. They're available from 3 a.m. to 10 p.m. Pacific time, seven days a week.