GeoServer: How do I get rid of error Unable to parse number For input string: "x"?

You are asking for WMTS tiles using the "REST" API - so when you request the getCapabilities document for your layer you will see something like:

<Layer>
<ows:Title>USA Population</ows:Title>
<ows:Abstract>This is some census data on the states.</ows:Abstract>
<ows:WGS84BoundingBox>
<ows:LowerCorner>-124.73142200000001 24.955967</ows:LowerCorner>
<ows:UpperCorner>-66.969849 49.371735</ows:UpperCorner>
</ows:WGS84BoundingBox>
<ows:Identifier>topp:states</ows:Identifier>
<Style>
<ows:Identifier>polygon</ows:Identifier>
<LegendURL format="image/png" xlink:href="http://localhost:8080/geoserver-2.17.2/ows?service=WMS&request=GetLegendGraphic&format=image%2Fpng&width=20&height=20&layer=topp%3Astates&style=polygon" width="20" height="20"/>
</Style>
<Style>
<ows:Identifier>pophatch</ows:Identifier>
<LegendURL format="image/png" xlink:href="http://localhost:8080/geoserver-2.17.2/ows?service=WMS&request=GetLegendGraphic&format=image%2Fpng&width=84&height=80&layer=topp%3Astates&style=pophatch" width="84" height="80"/>
</Style>
<Style isDefault="true">
<ows:Identifier>population</ows:Identifier>
<LegendURL format="image/png" xlink:href="http://localhost:8080/geoserver-2.17.2/ows?service=WMS&request=GetLegendGraphic&format=image%2Fpng&width=84&height=80&layer=topp%3Astates" width="84" height="80"/>
</Style>
<Format>image/png</Format>
<Format>image/jpeg</Format>
<InfoFormat>text/plain</InfoFormat>
<InfoFormat>application/vnd.ogc.gml</InfoFormat>
<InfoFormat>text/xml</InfoFormat>
<InfoFormat>application/vnd.ogc.gml/3.1.1</InfoFormat>
<InfoFormat>text/xml</InfoFormat>
<InfoFormat>text/html</InfoFormat>
<InfoFormat>application/json</InfoFormat>
<TileMatrixSetLink>
<TileMatrixSet>EPSG:4326</TileMatrixSet>
<TileMatrixSetLimits>
<TileMatrixLimits>
<TileMatrix>EPSG:4326:0</TileMatrix>
<MinTileRow>0</MinTileRow>
<MaxTileRow>0</MaxTileRow>
<MinTileCol>0</MinTileCol>
<MaxTileCol>0</MaxTileCol>
</TileMatrixLimits>
<TileMatrixLimits>
<TileMatrix>EPSG:4326:1</TileMatrix>
<MinTileRow>0</MinTileRow>
<MaxTileRow>0</MaxTileRow>
<MinTileCol>0</MinTileCol>
<MaxTileCol>1</MaxTileCol>
</TileMatrixLimits>
<TileMatrixLimits>
<TileMatrix>EPSG:4326:2</TileMatrix>
<MinTileRow>0</MinTileRow>
<MaxTileRow>1</MaxTileRow>
<MinTileCol>1</MinTileCol>
<MaxTileCol>2</MaxTileCol>
</TileMatrixLimits>
<TileMatrixLimits>
<TileMatrix>EPSG:4326:3</TileMatrix>
<MinTileRow>1</MinTileRow>
<MaxTileRow>2</MaxTileRow>
<MinTileCol>2</MinTileCol>
<MaxTileCol>5</MaxTileCol>
</TileMatrixLimits>
<TileMatrixLimits>
<TileMatrix>EPSG:4326:4</TileMatrix>
<MinTileRow>3</MinTileRow>
<MaxTileRow>5</MaxTileRow>
<MinTileCol>4</MinTileCol>
<MaxTileCol>10</MaxTileCol>
</TileMatrixLimits>
...

First you need to pick your TileMatrixSet or which projection you want. Then for each TileMatrixLimits you can get the Z value from the TileMatrix after the 2nd : and the min/max for X and Y from the Min/MaxTileRow/Col entries.