I had this same issue.
I found if you copy the tkdnd2.8 folder into the tcl8.6 folder it solves the error.
The tk8.6 folder is copied into the bunldle of software in the compile.
The folders above this are not copies.
Directory: C:\Users\Hu....\AppData\Local\Programs\Python\Python37\tcl
Mode LastWriteTime Length Name
---- ------------- ------ ----
d----- 25/07/2019 21:06 dde1.4
d----- 25/07/2019 21:06 nmake
d----- 25/07/2019 21:06 reg1.3
d----- 25/07/2019 21:06 tcl8
d----- 23/09/2019 18:14 tcl8.6
d----- 25/07/2019 21:06 tix8.4.3
d----- 25/07/2019 21:06 tk8.6
d----- 27/07/2019 20:40 tkdnd2.8 <=== copy this folder into tcl8.6 folder
-a---- 25/03/2019 21:25 184460 tcl86t.lib
-a---- 25/03/2019 21:25 7558 tclConfig.sh
-a---- 25/03/2019 21:25 773 tclooConfig.sh
-a---- 25/03/2019 21:25 826872 tclstub86.lib
-a---- 25/03/2019 21:25 124930 tk86t.lib
-a---- 25/03/2019 21:25 442812 tkstub86.lib
PS C:\Users\Hu....\AppData\Local\Programs\Python\Python37\tcl> dir tcl8.6
Directory: C:\Users\Hu....\AppData\Local\Programs\Python\Python37\tcl\tcl8.6
Mode LastWriteTime Length Name
---- ------------- ------ ----
d----- 25/07/2019 21:06 encoding
d----- 25/07/2019 21:06 http1.0
d----- 25/07/2019 21:06 msgs
d----- 25/07/2019 21:06 opt0.4
d----- 23/09/2019 17:51 tkdnd <=== I renamed it to tkdnd, not sure if this rename is necessary
d----- 25/07/2019 21:06 tzdata
-a---- 25/03/2019 21:25 21148 auto.tcl
-a---- 25/03/2019 21:25 128934 clock.tcl
-a---- 25/03/2019 21:25 7900 history.tcl
-a---- 25/03/2019 21:25 24432 init.tcl
-a---- 25/03/2019 21:25 22959 package.tcl
-a---- 25/03/2019 21:25 816 parray.tcl
-a---- 25/03/2019 21:25 33439 safe.tcl
-a---- 25/03/2019 21:25 5415 tclIndex
-a---- 25/03/2019 21:25 11633 tm.tcl
-a---- 25/03/2019 21:25 4860 word.tcl