Asp.Net Core Razor display an error message from if...else

Question

I have a button triggering OnPost action

 

<button asp-page-handler="PTWIssue" asp-route-idPTWNo="@Model.PTWNo" type="submit" class="btn btn-link text-info font-weight-bolder mt-2 mb-2" style="font-size:20px;" data-toggle="tooltip" data-placement="top" title="Issue the Permit to Work and send to Receiver">Issue Permit To Work</button>

On OnPost I have a check for string. I want to verify that there is no status "Awaiting" and then continue with OnPost. If the check fail, I want a simple message "Error on check" and the process to not continue. I have the following

 

var checkIsoCert = await (from a in _context.IsoCerts.Where(s => s.PTWNo == idPTWNo)
                                      join b in _context.IsoDetails on a.IdIC equals b.IdIC into TempData1
                                      from c in TempData1
                                      select new { c.IsStatus }).ToListAsync();

            string myString = "Awaiting";
            var matchingvalues = checkIsoCert.Where(x => x.IsStatus.Equals(myString));
            if(matchingvalues == null)
            {
                var ptwStatus = _context.PTWContents.First(s => s.PTWNo == idPTWNo);
                ptwStatus.PTWStatus = "Issued";
                await _context.SaveChangesAsync();
                return RedirectToPage("PTWForm", "PTWNoId", new { ptwNoId = idPTWNo });
            }
            else
            {
                //show an error message such as -You have not complete IsoCert-
            }

What I need is to display the message on error. How do I do that on Razor pages?