What is the largest risk for me if my portfolio is made of 99% REITS?

You should try to diversify your portfolio into other sectors, markets, asset classes. If interest rates go up, your entire portfolio will be impacted.

Concentration risk (on properties)
Diversify into other sectors and asset class
Buying Etf and index is a quick way to diversify

If your portfolio is made of 99% REITs, there is a risk that all the REITs will undergo a correction at the same time during a market downturn. To put it simply, your portfolio is not well diversified enough. I would recommend a diversified portfolio with different asset classes with low correlations.

The risk is that yr portfolio basically only expose to property. If there is any bad news on property (new ruling from gov, interest rate, etc...), the volatility will b very high.