This problem because your theme changed some line code of Shopify so please add one line with step by step:

Login admin page of your shopify

Open menu “Online store” and click “Edit code”, see image

Find file “product-template.liquid”, please check image.

Move to end of file and add this code edit your text after add to file product-template.liquid and save

<script>
var inkgo_custom_lang = {
    personalize: 'Personalize design',
    photo_title: 'Photo',
    required: 'Please enter all text fields and upload images',
    upload: 'Please upload file JPG, PNG, JPEG, GIF',
    save: 'Confirm',
    change: 'Change',
    rotate: 'Rotate',
};
</script>

Note: You only change text show in ”, for example ‘Personalize design’