The arm just slides across the record. - Technics SL-BD20D Turntable

I have not had a record player for 20 years. When I had one and I put the arm on the record and it slid across the record to the middle of the record the problem was it needed a new needle. It is the pointed part which contacts the record.
I found this manual info below hope it will help
Victrola VSC-550BT Operating Manual
https://www.manualslib.com/manual/1341772/Victrola-Vsc-550bt.html#manual
Victrola VSC-550BT Operating Manual: Troubleshooting Guide
https://www.manualslib.com/manual/1341772/Victrola-Vsc-550bt.html?page=15

Not enough weight on the arm, adjust the weight on the arm end. Can also be a build up of dust on the needle.

The needle (stylus) has fallen off the end of the cantilever that contacts the record surface. U need a new stylus assembly.

OR, it is bent at an extreme angle and slides across the record surface.

Does the needle actually contact the record? If not, you may be missing a "pad" under the record, or you may be simply missing the needle that needs to be there to follow the grooves.

It could be 3 different things. The first is check the needle make sure its not dirty. the second is make sure the weight is set correctly. And the third is did you take the tone arm off the protective part that holds the tone arm in place when moving or travelling .

This is often a problem when you are using a 78 needle on a 45 or 33 record. Some needles can be flipped, so check to see if yours is able to flip between the 78 and 33 setting. If not, you may simply need a new one.

Make sure the turntable is level.

Make sure the stylus is mounted correctly on the arm. With no record on the turntable, and placing a soft cloth under it, lift the arm and see if it slowly settles down or falls quickly. Depending on the make, arms can become sticky and lose their ability to "float." Re-adjust the weight dial if possible.

this old catridges have a small plastic where the neddle rest, but if its missing it couses to slide across the record, you will need to replace the whole assembly becouse they dont sell this little pieze by itself

There are a couple of reasons this is happening. First and formost gently lift the stylus arm and blow any and all dust off of the needle.You may even have to gently 'pinch' off any accumulated dust. Be careful since you don't want to pull the playing needle off.
This is a very common problem. If this does not help, you need a new stylus needle for the arm.

There can be a number of reasons for this.
Have you checked that the stylus tip has not broken off? this is a common cause and will require a new stylus.