Welcome to the group.
It sounds like your safety bar is engaged, preventing the top from lowering.
First, always raise and lower that RV when it's level. To do otherwise will risk damage.
Second, to lower, raise it back up slightly this time. There is (or should be) a cable near the switch that lowers the top. Pull the cable to disengage the safety bar and press the switch to lower. That should get you all the way down.
Look on the home page of this forum, down near the bottom of the page and find the red title bar that says LIBRARY. Under there you'll find a button next to something like Owners Manuals. Click that and look through the lists for a manual for a '77 model. If you don't see one for '77, just pick a year close to that. These didn't change a lot from year to year.
For more info, watch this series of about 32 videos on YouTube. This first one is on how to lower the top:
If you haven't bought this RV yet, there is a Pre-purchase checklist on this forum. Water damage is especially likely in a model as old as this one, so be aware. Look at this thread, about the 9th posting down the page for a detailed checklist.
The search function on this forum is your friend, and lots of folks here are ready to help.
Good luck and hope your '77 brings lots of joy!