Cannot backup Whatsapp Chat

When i try 'Backup Now' option in the chat settings of Whatsapp, it says "Not enough iPhone Storage. Free upto 50GB of storage space on your iphone to backup chats". I have tried all options to make space available on iphone, this is the best I could do but have Free space of 100 GB on iCloud. Please can you help a way to create chat backup on iphone/icloud wherever possible. Thanks.