Do you have any mods installed? I think there were some outdated mods affecting that functionality, like in this report: Northern Alliance, no Traders/Merchants?
Did you reach town inventory limit?
The error is definitely strange 
You can upload the savefile if you want, so that devs can take a look at it:
- Find the save folder for the saves you wish to upload. By default, they are located in
C:\Program Files (x86)\Steam\steamapps\common\Stonehearth\saved_games(assuming a default Steam install). - Zip the folder.
- Right click the folder (should be a long set of numbers).
- Left click on
Send toin the menu that appears. - Left click on `Compressed (Zipped) folder).
- (Optional) Rename the folder to something different than the default set of numbers.
- Upload the save.
- If the zip is less than 10 MB, you can upload it directly. Either use the upload button or drag and drop it into the compose window,
- If larger than 10 MB, upload it to a cloud storage site like Dropbox, Google Drive, File Dropper, etc. and post the sharing link here.