first things first:
1 - transmission fluid level?
2 - condition of fluid?
3 - last time the trans filter was changed?
beyond that, you might get lucky have a blocked fluid passage but there's a good chance the 1st gear clutch pack is shot.
the TH-400 doesn't change a whole lot over the years so the 1975 Chassis Manual will apply.
https://www.teambuick.com/reference/...ssis/index.php
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