This has started happening to me as well. Only started playing the Eco Life a Yesterday.
Before installing the expansions (Eco life and Get famous) I was able to travel with at least 3 Sims with no issues.
After expansions I can only select 2 Sims. When grouping my household (4) so they are all selected, the travel button is greyed out with a tooltip "Too many Sims selected"
If anyone is still seeing the issue (without mods) and are willing to share save files, instructions are here: