Normally the signature is derived based on past history and detected BT (breakthrough) events in that history.
Are you are following the instructions here to load up to 90 days of past activities?
Beginner’s Guide: Getting Started With Xert – Xert (baronbiosys.com)
If not, you can seed your starting signature on XO (xertonline.com) by entering 2-3 known values under Power Curve & Calculator on sidebar menu.
Or you can adjust your calculated signature by reviewing details for a recent activity, select Advanced MPA tab, change the signature entry at bottom, then Save/Lock which means use that signature moving forward.
Lots of newbie tips are posted in this thread –
Beginner questions - Support - Xert Community Forum (xertonline.com)
Also watch the Academy series videos as soon as you have time. This can be done while riding an indoor endurance workout using the Session Player by selecting a series under the Sessions Library. Or watch the parts in sequence from here – Xert Academy Videos - YouTube