Please follow the steps below to setup sticky header.
Edit the desired variant in the builder
Click on the header or logo and header's properties will be listed on the properties panel on the left. Make sure you have a background color set and Position is set to "Over" - https://cln.sh/biTF0B
Paste the Code available below in Settings - Custom CSS area - https://cln.sh/v3fU4o
div.tatsu-header {
position: fixed;
top: 0;
}