On one of my textbases, when I do a File/Import, the Accept Overrides and Accept Overrides & Update Validation Lists options are disabled.
Why?
On a different textbase, they're enabled...

The options are only enabled if the textbase has fields which have a validation list, mask, or range specified which have the following option checked in Edit Textbase Structure:

User May Override Content Validation

In order to enable the third option, the textbase must have a field with a validation list which has both of the following options checked:

User May Override Content Validation
User May Update Validation List with Override Value