I got a .bak file from one of our vendors. The instructions from them says that I should RESTORE WITH STANDBY and select "Tail-log backup" under options. Problem is that "Tail-log backup" is greyed out in SSMS. I am logged in as sysadmin and my SQL Server version is MSSQL14
A tail-log backup is only relevant for databases in full or bulk-logged recovery, and you can only perform a tail-log backup if you have access to an online copy of the same database.
If you're just restoring a copy of a backup of some other database in a new location (regardless if you're replacing an older copy), and all you have is a static .bak file, you don't need to check this box.
I think they just sent you instructions for how they restore this backup internally, maybe, but this portion is irrelevant to you if you don't have access to the original source database in order to take a tail-log backup.
(Also, I would ask your vendor for
RESTORE DATABASE commands, instead of a guide to pointing and clicking.)