I’ve enabled "Price Checking" in POS configuration setting and selected "Not allow below minimum selling price", but why the system still allows me to key in item price below minimum selling price?
In backend, you have enabled Below Minimum Selling Price option which allow user to bypass it.
ADMIN access right by default are able to by pass.
To disable it, go to Point Of Sales > Maintenance > Pos User Group Maintenance and then edit the group which you have assigned to your pos user.
Click Expand all and under Data Entry, uncheck Below Minimum Selling Price and then click on OK.
After that perform a synchronization.
By: CK 20200319, Jacky 20200320 
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