Configure the option "Maximum number of undo steps" (File | Options)

Here you can configure how many levels of undo are possible in PlanMaker.

Undo

Registry Hive	HKEY_LOCAL_MACHINE or HKEY_CURRENT_USER
Registry Path	SOFTWARE\Policies\SoftMaker\PlanMaker 2018
Value Name	Undo
Value Type	REG_DWORD
Default Value	99
Min Value	1
Max Value	999