AutoCAD LT 2017 for Mac

iMac OS Sierra Ver 10.12.3

I'm a engineer & been using AutoCAD LT on iMac for a month or so with no problem then started getting this error below when try to open AutoCAD files sent to me by other consultants.

AutoCAD LT Message

Cannot find the specified drawing file.

Please verify that the file exists.

Anyone else had this problem & maybe know the cause and/or a solution ?

Thank you

JM