When you say you can’t delete all data, are you saying there are some records which “refuse” to be deleted? The checkbox at the left of the primary field’s column name will select all records in that view and from there you can right-click to delete them all.
I think you may be referring to Collaborator-type fields. If a collaborator is removed from a base (or in your case, if you use a template that has collaborators filled in who don’t have access to your base), the field value will still be displayed albeit a little grayed out. You can of course click that cell or column to clear it.
I can interpret this statement as one of three things:
You literally cannot add a row: You’re most likely looking at a Grid View which has been grouped by a calculated field. There should be a round add button at the bottom right of your screen to add a new row.
New rows show
ERROR in a Formula field: That most likely means the formula is relying on another field’s value which is currently blank. Fill in the fields for that row and the error should go away.
Some columns have a pinkish caution symbol next to their name: That means the column/field in question is a Rollup or Lookup field that is relying on a field which used to be a Link To Another Record field which has either been converted to another field type (breaking the link) or deleted. Set up a proper link between records and adjust the Rollup/Lookup field as necessary.