Use lookup value in column formatting.

avytrykh you've got an elm type of span AND an elm type of button both operating on the same item. That's the main cause of your problem; I would remove that span with the if currentfield txtcontent. The following will work (change flow ID and the column name to yours):

	{
  "$schema": "https: //developer.microsoft.com/en-us/json-schemas/sp/column-formatting.schema.json",
      "elmType": "button",
      "txtContent": "Accept",
      "customRowAction": {
        "action": "executeFlow",
        "actionParams": "{\"id\":\"0eb7e84b-4158-47b0-b19e-aff0aa26af3d\", \"headerText\":\"Sign Off\",\"runFlowButtonText\":\"Request Sign Off\"}"
  },
      "style": {
        "background-color": "#2ca1e8",
        "color": "white",
        "visibility": "=if([$Project.lookupValue] == 'G-HERC', 'visible',  'hidden')"

      }
    }

Rob
Los Gallardos
Intranet, SharePoint, Website and Power Platform Manager (and classic 1967 Morris Traveller driver)