Hello,
package: forest-express-sequelize@8.5.12
The type definition of SmartFieldOptions missed isSortable
PR: Add "isSortable" to SmartFieldOptions interface by JustinMartinDev · Pull Request #1012 · ForestAdmin/forest-express-sequelize · GitHub
Hello @Justin_Martin,
Thank you for your PR. 
A colleague will take a closer look at this topic. There’s indeed something weird as the documentation explains how to implement it. We have to be careful, as some customers could think that setting isSortable to true will be enough to make it sortable (witch is false with forest-express agents).
Kind regards,
Morgan
Hi @Justin_Martin 
Thank you for your contribution.
We merged it and it should be released now.
Please install the last version (8.5.13) and let me know if that help.