Make sure that you have an authentic Samsung S Pen. There are many aftermarket ones that look the same, but do not have the Bluetooth ability the legitimate S Pens seems to need.
I just had to replace my lost Tab S7 S Pen and noticed many low cost replacements listed on ebay and Amazon, but I saw the complaints about them. Ordered an authentic Samsung S Pen new in box, and it connected to my Tab S7 as soon as it had the teeniest charge.
