Too many WebGL contexts. Unable to add the layer
The 3.x version of the JS API creates one WebGL context per vector tile layer added to a workbook, and Chromium has a limit of sixteen WebGL contexts. Mobile browsers have a limit of eight WebGL contexts.
Use a raster basemap, rather than a vector tile basemap.
Additionally, the 4.x version of the JS API utilizes WebGL contexts differently so that this limit will not be reached as quickly. Consider creating a map in the new Map Viewer, or create an Instant app or web experience instead of using Web AppBuilder.