Slice by Surface would not work for me either but as you know with TC there are more ways to accomplish an edit...
Method I used.
1. Select both on the solids and make a copy (I entered 500 in the X delta)
2. Select the Profile you created trying to use as a slice line.
3. Shell Solid by -1500
4. Select Solid Subtract and engage the"Dont Remove Subtrahend"
5. Select the stern solids and select the shelled profile and subtract.
6. Move the copies back inline with the others (-500 in the X Delta)
7. Select Sold Subtract and engage the"Dont Remove Subtrahend" again
8. Select the taller copies and subract them with the others..
Attached my edit below:
V2019 Plat 64bit, Lenovo P72 Laptop, Window 10 Pro for Workstations, Intel Xeon E-2186 CPU @ 2.90 Ghz (6 cores/12 threads), 32GB RAM, 512GB & 1TB SSD's, Nvidia P5200 c/w Max=Q Design GPU, Display UHD 3840 x 2160 pixels
TurboCAD user since V3 and Turbocad 3D V1.