Can I add my entry in Form 2555 line 18(e) "Number of days in US for business" if I am on vacation? Entering 0 didn't work, it said 1 is the minimum value. Any idea?

It depends. You wouldn't enter this on the form anyway.  You will enter this in the program.

To enter Foreign Earned Income -

• Click on Federal 
• Click on Wages and Income  
• Click on I'll choose what I work on (if shown)
• Scroll down to Less Common Income
• On Foreign Earned Income and Exclusion, click on the start or update button
• Scroll through the entries until you reach a section that says Enter information about US Travel
• Here is where you will specify how many days you spent in the US for vacations.
• Here is what the screen will look like.
• Leave the the last two columns blank if you were only vacationing and not in the US for business.

The issue persists in Turbotax Premier 2022 and seems to be a glitch. What I did is that I left the "number of days present", "number of days on business", and "income earned" fields blank. That allowed me to continue onwards. When I got to the federal review section, it then showed me the 2555  form itself allowed me to insert the "number of days present" without forcing me to include "1" as the minimum value for "number of days on business". 

Using TT Deluxe 2022 I’m having the same problem reported below.  For Form 2555 line 18(e), number of days in U.S. on business, TT requires entry to be at least 1.   This applies in interview, in form, and in federal review.  But the true correct value is 0 because presence in US was only for family visit.  No business.  This certainly seems to be an error in the TT program.  Please fix, or explain how to enter zero.