No, with Shoal you'll never get back less than the amount you put in. Shoal offers a fixed-term savings product (also known as a fixed-saver). Shoal does not offer an investment product at present, and therefore your capital is not at risk.
When you create a Savings Pot in the Shoal app, your savings:
Earn a fixed rate of return over your selected Savings Pot term. You will get your savings back, plus your return rate at the end of your Savings Pot term. We'll also show you how much you'll get back before you finish creating your Savings Pot. No surprises.
Your active Savings Pots are FSCS-protected.