can't log starting weight

You must use the Web version, not the mobile apps, to update your starting weight.

Navigate to APPS > Tickers > Update your weight data. Enter your desired starting weight and click Save Changes. That will update the oldest date/weight in your profile.

Alternately, navigate to MY HOME > Check-in > Edit Previous Entries.
a. Enter a new value in the Starting Weight text box and click Change; or
b. Scroll down and click on the highest page number to take you to the oldest date/weight entry. Once you find your oldest date/weight entry, under Add New Entry, enter the new date/weight and click Add New Entry. That will replace the oldest date/weight with your new one.
c. If you want to keep all of your historical data, Add New Entry at least one day older than the oldest date/weight.