You can use the following action:
command-sequence inset-modify tabular set-pwidth 10cm; layout standard; inset-modify tabular set-pwidth
To run it directly, make sure your cursor is in the cell in the table, press alt+x for the mini-buffer, paste it in, and hit return.
If you like this, you can make a shortcut by going to the following:
Tools > Preferences > Editing > Shortcuts
However, I hope that you don't like this. The best thing to do is to report this as a bug. This is a hack and you should not rely on it.
The reason for this behavior is the following:
The mechanism that determines whether you can modify the layout is
ForcePlain. For more detail, search for that word in
Help > Customization
It is hardcoded for table insets and is determined as follows (as shown in
bool InsetTableCell::forcePlainLayout(idx_type) const
Thus, the hack works as follows: it changes to a fixed width, changes the layout to standard, then removes the fixed width.
If you want a list inside a table in the future (asked in a comment to the question), the correct way is to make a fixed width and then change the layout.
Note that to edit the width graphically, place the cursor in the table and go to
Edit > Table Settings