I'm practically pulling my hair out on this!
When I select one line in a sketch as shown here I have the option to convert it to a construction line.
when I select 2 or more lines the option to change them all to construction lines disappears.
This just started yesterday. Normally I can select as many lines as I want to convert and do them all at the same time.
Oh look at that. I just answered my own question. Somehow I must have clicked the drop down arrow and hidden the option. it stays hidden until I click it down again.