Activate Custom Script for SharePoint 2019

When installing the solution on your SharePoint on-prem, it's possible that some of the required features to add extra functionalities are not activate, below is a list of the known issues.

1. If you don’t have the Solutions Gallery available to install your Classic theme on your SharePoint site, the cause of that is because you have the Custom Script disabled and it is required to enable it to have access to that feature.
2. When you’re applying the Full-Color Experience with your BindTuning modern theme, if you don’t have the custom script activated, SharePoint cannot upload the SpColor file which contains styles for elements on the modern Experience.

To make that you need to execute the below steps that will require you to have access to your SharePoint server. Below are the steps that you need to follow:

1. Login to your SharePoint Farm SharePoint 2019 On-Premise
2. Open SharePoint 2019 Management Shell as administrator
3. Execute the following PowerShell command:
   (Get-SPSite -Identity "https://Server/sites/SiteURL").DenyPermissionsMask= [Microsoft.SharePoint.SPBasePermissions]::EmptyMask
4. The solutions gallery will be visible on your Site Settings.

For reference regarding How to Enable Custom Script in SharePoint Online using PowerShell, please read the following article.