Intermediate checks Testing Lab

You said you had an internal Audit. I can’t tell if this was a third-party consultant on your behalf or the accreditation body, assessing you for accreditation. If it was an accreditation body, you must comply with their technical requirements for traceability and measurement uncertainty.

To meet ISO 17025 requirements as a testing laboratory, you must ensure your method is valid for the purpose, and reliable by meeting all the technical requirements including:
1. Verifying that the equipment used has the performance specification required (eg resolution/gauge units), 
2. Proving through experimentation that the method is valid –i.e. accurate to the extent required.
3. Having metrological traceability through an unbroken chain of calibrations.

Accuracy is a qualitative description that you need to represent as Uncertainty. This has two quantitative components, being bias (a measure of trueness due to any systematic error) and a quantitative measure of the spread of results (a measure of precision due to random error of measurement).

From what you shared, it seems the Auditor's findings are valid. You said:

Based on the repeatability of the final reading in MPA for (UTM) i made 1.5x expanded uncertainty as my UCL/LCL

You must take both systematic and random errors into account. Not just repeatability (random error) and you need to show that the MU estimate is acceptable. (e.g. it cannot be smaller than the machine uncertainty. You need the standard uncertainty of each contributing device as a percentage (relative) and then represent the final expanded uncertainty in the units.

he insisted to have calibration of slip gauge.

Yes to ensure metrological traceability this is required. There are times when you can achieve this with an in-house secondary calibrator device, providing traceability to an externally calibrated device. The external calibrations must always provide the MU of the unit tested and reported.

For more information, have a look at a previously answered question at https://community.advisera.com/topic/measurement-uncertainity/

The ISO 17025 toolkit document template: Equipment and Calibration Procedure at https://advisera.com/17025academy/documentation/equipment-and-calibration-procedure/

The ILAC P10:07/2020 ILAC Policy on Metrological Traceability of Measurement Results is available from https://ilac.org/publications-and-resources/
The article: What does ISO 17025:2017 require for laboratory measurement equipment and related procedures? at https://advisera.com/17025academy/blog/2019/07/25/iso-17025-measurement-requirements-of-the-standard/