Looks like a bug with Weight Classes

[Zakawyka]

Try to setup Weight Class
The manual says "
Whatever you have set to 1.00000 is your base weight class. Any other weights in there need to be a factor of your base class."

Try to delete POUND which is not the base weight - error says " App Error Warning: This weight class cannot be deleted as it is currently assigned as the default store weight class! "
Nonsence.

[llegrand]

the warning error has told you the issue.  -  If you want a different weight class to be the default you need to set it to 1.0000.
Which ever wt class is 1.0000 becomes the default.  System will not let you delete the default class. 

Once you set a new weight class to 1.0000 for default  then you can delete any other one(s) you wish.

If you have more than one class,  you need to set the parameters for it as a function of your default class

[Basara]

Hello.
Please do not remove the default weight classes. Even if you don't use it.
To change the Store weight class please check your Store Settings https://abantecart.atlassian.net/wiki/spaces/AD/pages/16121910/Weight+Classes#WeightClasses-WeightClassesconfiguration

[Zakawyka]

Dear Sirs,

POUND was neither Base nor Store main class, KILO was the main one. So I try to delete not the main class, and the software insists that is it the main one.
I have read the instruction regarding differences in settings. Store Set class was set to KILO

Pity that none of you bothered to have a look on the screenshot attached where POUND cleary marked and its weight is 2 something, not 1.0000000

[llegrand]

And more's the pity you assumed we didn't look at it!.   However,  we also assumed you could see the edit button on the right side.
Seems we both might have been in error.

Attached screen shot so you can see ALL of the  fields. 

Hopes it helps you.