This prompt means that you need to provide master password of Enpass data present on respective cloud (not your cloud login password). The reason for getting this message might be:
You are using Enpass on multiple devices with different master passwords and then syncing the keychain over cloud.
Recently you have changed Enpass master password on one of your device and then synced the keychain with cloud.
NOTE: When you change your master password of Enpass on one device, it gets changed there and as well as for the Enpass file on your cloud account with which sync is ON. Master password of Enpass will not be changed on rest of the devices syncing with that cloud because they are not aware of new password.
Since Enpass data on cloud is now encrypted with new password, so your other devices will not be able to sync with the data on cloud and will show this message. Now you need to enter your new/changed master password (only once) to enable sync. As soon as you enter the changed password, your sync operation will resume.