Make sure that the cuff connector is properly inserted in the hole located on the side of the device and also check that nothing is squeezing the connection tube.
Make sure that the battery is sufficiently charged and the cuff is properly positioned. See How to place the cuff properly.
*credit to Sylvain of iHealth Labs EU
https://helpcenter.ihealthlabs.eu/hc/en-gb/articles/115005420185-iHealth-Clear-BPM1-The-cuff-does-not-inflate