Hope you are doing well and sorry for the trouble. As described you don't see the letters when you write, so you can't fill out the form.
Is this a behavior with a particular PDF form or with all the PDF forms? Please try with a different PDF form and check.
Also, please make sure that the text color is not set to white. Please open the PDF form in Adobe Acrobat DC > Go to Tools > Prepare Form > Double click on the text field to open text field properties > Under 'Appeareance' tab set text color to Black