Look at the GEDCOM file in a text editor. The first line should be:
Followed by a:
The VERS probably indicates 5.1 or something less than 5.5
Change this to 5.5 and try the import.
You will probably get a bunch of errors that tell you that some of the GEDCOM tags were not correct.
Some of them probably are true because Generations was not very good at using GEDCOM correctly. Some of them are true because FTM does not support GEDCOM completely.
But most of the important/basic data will be loaded to FTM. You will then need to go thru all of the errors and see what has been droppped or not added to FTM and add them correctly.