Hi, Welcome to JustAnswer.com, I am glad you are here! My name is Eric & I will endeavor to answer your question.
Wow a Stanley, Those are few and far between these days! I have two things to tell you. The first is going to be about your problem and the second is going to be about Stanley Garage door openers.
The symptom you are describing is indicative of a failure in your safety sensor circuit. When the safety sensor circuit is malfunctioning, the door will open as normal with the remotes, keypad (if you have one) and wall buttons however, will not close with the remotes or keypad and will only close by keeping constant pressure on the wall button.
The problem is most certainly in the safety sensor circuit. Where though...?
Here are the options:
A. The safety sensors are dirty. - Wipe lenses with soapy water and a clean soft cloth
B. The safety sensors are misaligned (This is the most likely problem and the most common issue with Garage Door openers). - Follow the safety sensor alignment instructions in your owner's manual. If you do not have a copy of your owner's manual I can see if I can provide one for you if you will give me the manufactures name and exact model number.
C. There is a broken or disconnected wire between the motor and the safety sensors. - Carefully follow the wire path and do a visual inspection to ensure a continuous connection.
D. You have a defective sensor which requires replacement.
E. In rarer cases still, if all of the above has been attempted yet the problem persists, then there has likely been a failure in the logic board which would then mandate it's replacement.
I hate to be the bearer of bad news however, garage door openers under the name of Stanley have not been made in over 16 years. Even then there were two companies that were manufacturing OEM branded Stanley Garage doors & Openers and both of these companies went bankrupt long ago.
Stanley does not manufacture parts nor do they offer service or support for these products.
If the solution to your problem is not one of the easy and cheap ones as described above, I recommend that you bite the bullet and buy a new opener. Even if you were able to get your Stanley opener working, the life expectancy of a garage door opener made in the early to mid 1990s is only 10-12 years so (In my opinion) spending any more time or money on trying to revive your deceased Stanley opener, is just throwing good money after bad especially when a decent new opener can be purchased for around $150.00.